394 C. 0. WHITMAN. 



The second great feature common to the embryos of both 

 Annelids and Vertebrates is the metameric division of the 

 body. In the Annelids the division into somites follows closely 

 in the wake of concrescence, and the two processes are funda- 

 mental and invariable in sequence. Noav, when we see that 

 the vertebrate embryo arises from a germinal ring, and that 

 soon after the formation of the embryo begins, the metameric 

 division sets in and progresses in the same direction as in the 

 Annelids, there is certainly good ground for inferring that the 

 phenomena are fundamentally the same in both groups of ani- 

 mals. Since, in the one case, concrescence and metameric 

 division are conceded by all to be the two grand formative 

 processes associated in invariable sequence, it is difficult to be- 

 lieve that, in the other case, the second process has been pre- 

 served, while the first in order, and perhaps in importance, has 

 been entirely suppressed. 



The law set up by the elder Milne-Edwards, according to 

 which fresh somites are intercalated b&tween the last formed 

 somite and the hind end of the body, does not appear to me to 

 hold good in the case of embryos that arise by concrescence. 

 In Clepsine it is perfectly clear that the hindmost segment is 

 the last formed segment ; and the investigations of Kowalevsky, 

 Hatschek, and others appear to demonstrate the same for the 

 Chsetopods. The germ-bands lengthen at the expense of pro- 

 liferating cells ; and the additions are made to the hind extre- 

 mities, so that these are always the youngest portions of the 

 embryo. The theory of intercalation can therefore be upheld 

 only by supposing that the proliferating cells represent the 

 hind end of the body, which they do only prospectively. But 

 there is still another objection to this theory, which was pointed 

 out by KoUiker.^ In Clepsine the somites are not added one 

 by one as fast as the material is furnished by the proliferating 

 cells. The metameric division begins only after the materia, 

 for the greater number of somites is already present, so that 

 no one of the successively formed somites, except the ultimate, 

 can be regarded as the youngest portion of the embryo. 



' KoUikcr, ' Observationes dc prima insectorura gencsi,' Zurich, 1S42, 



