20 METAMORPHOSES Of MAN 



this point some slight differences are occasionally 

 exhibited. In certain eggs with a large vitellus, a 

 portion of the yolk escapes segmentation ; but in all 

 animals the result of this change is the formation of a 

 primitive organized layer which embraces the yolk, 

 and has been termed the blastoderm* 



These first traces of organization have hardly ap- 

 peared when all resemblance between the various 

 germs ceases. The germ becomes an embryo, and at 

 the very outset exhibits the fundamental characters of 

 the group to which the new being will belong. Up to 

 this period both Vertebrates and Invertebrates have 

 travelled together along the developmental road ; but 

 now they part, never to meet again. Henceforth the 

 two great divisions of the animal kingdom, the two 

 sub -kingdoms will remain entirely separate. 



In Vertebrata, the organic elements — cells and gra- 

 nules — accumulate and arrange themselves, at one 

 point in the blastoderm in the form of a minute spot, 

 which is circular at first. This spot is the germinal 



.scar is obscure. This cicatricule is according to him the real egg ; 

 the remainder, or yolk, being merely an accessory body which 

 masks and obscures the phenomena of the earlier stages of existence. 

 * The phenomena observed by M. C. Siebold in the planaria 

 (Manuel d'Anatomie comparee), and by Messrs. Koren and Daniel- 

 sen in the pectinibranchiate mollusks, form two remarkable excep- 

 tions to the general rule (Ann. des Sciences naturelles, 1852). But 

 Dr. Carpenter has already shown that in the case of Purpura 

 lapillus, these naturalists have been in error. The peculiarity of 

 development in these mollusks is, doubtless, still exceptional ; but, 

 as regards the general processes of segmentation, and the expulsion 

 of the globules (directive vesicles, Cams and Carpenter), they come 

 under the general law. — (" On the Development of the Embryo of 

 Purpura lapillus." — Transactions of the Microscopical Society of 

 London.) 



