26 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. 





Uti.,1. ill (A) 



Ceratodua and (B) / 

 - dosiren. (A after Si-mon, 

 1893.) 



Figi. 2-6 are vieu 

 pole : PigK. 1, 6 an> I 7 I 



becomes thus divided into 

 eight practically equal blas- 

 tomeres. 



A latitudinal furrow 

 then develops about 45 

 above the equator, so that 

 the egg now consists of eight 

 micromeres round the apical 

 pole and eight macromeres. 



After this stage seg- 

 mentation usually becomes 

 irregular although some- 

 times two additional latitu- 

 dinal furrows make their 

 appearance in succession so 

 that the egg consists of four 

 tiers - each of eight blasto- 

 meres. Eventually, as seg- 

 mentation proceeds, a bias- 

 tula is formed of the type 

 shown in Fig. 15, A 7. 



The segmentation cavity 

 first appears about the time 

 of the fourth cleavage as a 

 small chink. It rapidly 

 expands and in the blastula 

 figured (Fig. 15, A 7) it is 

 of large size. 



In Lepidosiren (Fig. 

 15, B) the egg measures 

 usually between 6 '5 and 7 

 mm. in diameter. It is free 

 from pigment in correlation 

 with the fact that it de- 

 velops in a burrow shaded 

 from the action of light. 

 In the region of the apical 

 pole is a whitish cap in 

 which the yolk is in very 

 minute particles while else- 

 where it is in laruf cnarse 

 granules, 



The first two furrows 

 Ki-. !;", 1'. 'J.-unl :> are meri- 

 dional and at ri.irlit angles 

 to one uimlher. The third 

 set nf furrows ( I-'i-. 1 5, I- -I ) 



are vertical though occa- 



