EARLY STAGES 



621 



rise to the ectoderm, the yolky cytoplasm on the vegetative 

 side will give rise to the endoderm, and the crescent of granular 

 cytoplasm between the two at the hinder end will give the meso- 

 derm. The first cleavage (Fig. 479) is vertical and forms two 

 equal cells. The second is also vertical, at 



an.i 



Fig 



vr.m. 



Ova of 



Branchio stoma. 



B, after 



A , Unfertilised 

 fertilisation. 



right angles to the 

 first, but not quite 

 equal, the third nearly 

 equatorial, dividing 

 each blastomere into 

 a rather smaller upper 

 half and a rather 

 larger lower half. The 

 cells do not meet in 

 the middle, so that at 

 this stage they form a 

 ring. Division con- 

 tinues, and up to the 

 256 cell stage, which 

 is reached at four 

 hours, it is synchron- 

 ous, that is, all the cells 

 divide together. The 

 constituent cells of an 

 early embryo are called 

 blastomeres. Eventu- 

 ally there arises a 



Fig. 479. — Early stages of 

 the cleavage of tl 

 of Branchiostoma 



Fertilised ovum ; cL, clear 

 cytoplasm which after cleavage 

 will be situated in the ectoderm 

 cells ; »«., granular cytoplasm 

 which will be in the mesoderm 

 cells ; y., yolky cytoplasm 

 which will be in the endoderm 

 cells ; 2., first cleavage — stage 

 of two blastomeres ; 3., four 

 blastomeres ; 4., eight blasto- 

 meres ; 5., thirty-two blasto- 

 meres. 



an.p., Animal pole ; chr., , . , , , -, j 



chromosomes of the two slightly pear-snapecl 



pronuclei, which have , 



met; cL, clear cyto- strUCtUrC, knOWn aS 

 plasm; gr., granular . . 



cytoplasm ; nu., nucleus thc blastula, thC CaVlty 



^pi^p.b.2, filJsf ''and of which is the blaStO- !:^:dicated by the^arrow^ m No, 

 second polar bodies 



The figures are so posed that the 

 side wnich will, in the adult, be 

 dorsal is above, and that which 

 will be anterior is to the left ; the 

 horizontal axis of the adult is 



I. 

 The animal pole of the ovum. 



v.m., viteUine membrane; CCelc. ItS Wall COUSistS '^^r.^^?^ J^J^^tJ^''''"^^ ^^" 

 y., yolky cytoplasm. 



body, IS antoro-ventrai. 



of a single layer of cells, 

 which differ somewhat m different regions. The ventral and 

 anterior part is composed of small cells derived from the clear 

 cytoplasm of the ovum, the dorsal cells are larger and more yolky; 

 a crescent around the posterior end is made up of smaller cells 

 with granular cytoplasm (Fig. 4S0, 2 and 3). 



GASTRULATION 



There now sets in a process known as gastrulation. 1)\- which 



