Vietnam or Thailand ? Vote for the TOP Country of the Week !
Updated: May 24, 2025
The reduction of the radii to four was accompanied by a further differentiation of the fin-rod, its transverse segmentation into upper and lower halves, and the formation of the zone of the limb, which is composed originally of three limbs before and behind in the higher Vertebrates. There is also a complete agreement between the fore and hind limb in the stem or shaft.
"The logic of impecuniosity will doubtless accomplish more than the dissuasion of friends. Microscopic inspection of red and white corpuscles, of virus, tissues, protoplasm and chlorophyl is probably very interesting to lovers of microbes, and students of segmentation, but such abstract pursuits appertain to purple and fine linen. A profession means much; but ability to practise, infinitely more.
Thus, in the highest Annulosa the exo-skeleton and the muscular system never lose all traces of their primitive segmentation; but throughout a great part of the body, the contained viscera do not in the least conform to the external divisions.
H. Spencer seems to deny serial homology to the mollusca, but it is difficult to see why the shell segments of chiton are not such homologues because the segmentation is superficial. Similarly the external processes of eolis, doris, &c., are good examples of serial homology, as also are plainly the successive chambers of the orthoceratidæ.
Nauplius-brood. Ord. 1. Ceratometopa. Nauplius with frontal horns. Ord. 2. LEIOMETOPA. Nauplius without frontal horns. Sub-class II. HEMISCHISTA. Segmentation not complete. A. Nototropa. Embryo bent upwards. Ord. 3. Protura. The tail is first formed. Ord. 3. Saccomorpha. A maggot-like larva-skin is first formed. B. Gasterotropa. Embryo bent ventrally. Ord. 5. Zoeogona.
It is therefore very interesting to find that in the other crustacean before mentioned, the squilla, the segmentation of the body is more completely preserved, and even the first three segments, which go to compose the head, remain permanently distinct. Such an obvious and unmistakeable serial repetition of parts does not obtain in the highest, or backboned animals, the Vertebrata.
A "true and natural system" of the Crustacea to be in accordance with the sequence of the phenomena would have to take into account in the first place the various modes of segmentation, then the position of the embryo, next the number of limbs produced within the egg and so forth, and might be represented somewhat as follows: Sub-class I. HOLOSCHISTA. Segmentation complete. No primitive band.
Until recently it was regarded as a general rule that, by the partial segmentation of the vitellus a germinal disc was formed, and in this, corresponding to the ventral surface of the embryo, a primitive band. According to Mr.
We might urge that, according to this proposition, provisional organs as the first produced must exceed the later-formed permanent organs in importance. But let us stick to the Crustacea. In Polyphemus Leydig finds the first traces of the intestinal tube even during segmentation. In Mysis a provisional tail is first formed, and in Ligia a maggot-like larva-skin.
As these two nucleii finally come together their membranes disappear, and the chromatic material comes to lie freely in the egg, the male and female chromosomes, side by side, but distinct forming the segmentation nucleus. The egg plainly now contains once more the number of chromosomes normal for the cells of the animal, but half of them have been derived from each parent.
Word Of The Day
Others Looking