122 



COMPARATIVE ANATOMY 



CHAP. 



blastomeres into 8, of which the 4 animal portions form the animal part of the germ, 

 and the 4 vegetative portions its vegetative part. 



In the holoblastlo alecithal gei-m the 8 blastomeres are of equal size, and the 

 third plane of division shows itself outwardly as a strictly equatorial circular furrow 



(Fig- 90. ■^)- ,. ., . , 



In holoblastio telolecithal germs each of the 4 blastomeres divides into a 

 smaller animal blastomere, consisting almost exclusively of formative yolk, and a 

 larger vegetative blastomere containing the nutritive yolk and a small quantity of 

 formative yolk. The greater part of the latter lies towards the animal pole of 

 the blastomere and contains the nucleus. Each vegetative blastomere of the germ of 

 8 blastomeres thus repeats the structure of a blastomere of the germ at the 4-blasto- 

 mere stage. The more considerable the quantity of nutritive yolk in the'germ, 

 the larger is the vegetative blastomere as compared with the animal. The smaller 

 blastomeres are called mioromeres, the larger macromeres. The division of the 4 

 blastomeres into 4 micromeres and 4 macromeres looks as if the former budded from 

 the latter (Fig. 90, B, 0). 



In the holoblastio centrolecithal {Geryonid) germ, after the appearance of the 

 third plane of division, which is here strictly equatorial, the 8 blastomeres are of 



m 





Fig, 90.—^, F, Diagrams showing tlie 8-ljlastomere stage of different eggs. 



equal size. Each blastomere has a peripheral layer of formative yolk, with a nucleus 

 imbedded in it, and a mass of nutritive yolk towards the centre of the spherical germ 

 (Fig. 90, D). 



The germ of the Ctenophora departs from the usual arrangement of the telolecithal 

 holoblastio germ, since the third process of division is not equatorial. Each of the 

 4 blastomeres divides, in a plane slantingly meridional, so that the germ consists 

 of 8 approximately equal blastomeres, of which 4 lie rather nearer the animal pole 

 than the other 4. The 8 blastomeres are grouped round a central space, which 

 lies in the principal axis of the germ. Each blastomere shows the structure of the 

 original Cteiiophoran egg, the fonnative yolk lying chiefly at the animal pole of the 

 blastomere, and the more considerable nutritive yolk forming its larger vegetative 

 part. 



In the meroblastio telolecithal germ, in consequence of the great development of 

 the nutritive yolk, the " equatorial " division takes place quite near the animal pole. 

 Exteriorly this division is visible as a cu-cular furrow (polar circle) which divides the 

 4 prominences of formative yolk into 4 smaller central prominences with nuclei, and 

 4 peripheral prominences also with nuclei. The 4 central and the 4 peripheral 

 prominences are not completely separated beloAV the surface, either from each other 

 or from the subjacent nutritive yolk (Fig. 90, F). 



In the meroblastio mesolecithal germ the 8 central masses of formative yolk, 

 each provided with a nucleus, are not separated from the great mass of nutritive 

 yolk surrounding them (Fig. 90, H). 



No general rule can be given for further divisions. Many meridional, equatorial, 

 and cross divisions follow. The determination of the processes in detail is the more 



