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In his excellent work, On the structure of the Heart in the Amphibia , Carl Rabl has shown how easily these curious cenogenetic facts can be explained by the secondary adaptation of the embryonic structure to the great extension of the food-yelk. The embryology of all the other parts of the vascular system also gives us abundant and valuable data for the purposes of phylogeny.
The embryology of the modern types confirms this idea, for here we find an epitome of their geological history. The embryo of the present Star-Fishes recalls the Crinoids; the embryo of the Crab recalls the Trilobites; the embryo of the Vertebrates, including even that of the higher Mammalia, recalls the ancient Fishes.
As the work proceeds, Professor Haeckel now and again calls the attention of the entire class to some particular phase of the subject just passing under their individual observation, and in the most informal of talks, illustrated on blackboard and chart, clears up any lurking mysteries of the anatomy, or enlivens the subject with an incursion into physiology, embryology, or comparative morphology of the parts under observation.
One is written in the rocks, one in bone and muscle, while the third is recorded in the evanescent and changing pages of embryology and metamorphosis. Yet each tells the same story. Each tells of a history of this machine from simple forms to more complex. Each tells of its greater and greater differentiation of labour and structure as the periods of time passed.
On the principle of successive slight variations, not necessarily or generally supervening at a very early period of life, and being inherited at a corresponding period, we can understand the great leading facts in Embryology; namely, the resemblance in an individual embryo of the homologous parts, which when matured will become widely different from each other in structure and function; and the resemblance in different species of a class of the homologous parts or organs, though fitted in the adult members for purposes as different as possible.
But when we recognise the causal correlation of ontogeny and phylogeny expressed in this law, the wonderful facts of embryology are susceptible of a very simple explanation; they are found to be the necessary mechanical effects of the evolution of the stem, determined by the laws of heredity and adaptation.
First, the comparative anatomy of all the different groups of animals; second, their comparative embryology; and third, their palæontological history. Each source has its difficulties or defects. But taken all together they give us a genealogical tree which is in the main points correct, though here and there very defective and doubtful in detail.
While there are many "missing-links," owing to the disappearance of the forms which formed the connection, still there is sufficient proof left in the existing forms to satisfy the fair-minded inquirer. The facts of embryology alone are sufficient proof of the ascent of Man from the lowly forms.
No one has borne warmer testimony to the influence Dollinger has had in this indirect way on the progress of our science than the investigator I have already mentioned as his greatest pupil, von Baer. In the introduction to his work on embryology he gratefully acknowledges his debt to his old teacher. "Among the most fascinating of our professors was Oken.
The coeloma or body-cavity has some very important and distinctive features in the Amphioxus. The embryology of it is most instructive in connection with the stem-history of the body-cavity in man and the other vertebrates.
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