26 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



Fiu. 15. — Segmentation in (A) 

 Ceratodus and (B) Lepi- 

 dosiren. (A after Senion, 

 1893.) 



Figs. 2-5 are views of the apical 

 pole: Figs. 1, 6 and 7 are' side 

 views. 



Ibecomes thus divided into 

 ■eight practically equal blas- 

 Itorueres. 



A latitudinal furrow 

 is 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- 

 Imentation usually becomes 

 ■irregular although some- 

 Itimes two additional la titu- 

 Idinal furrows make their 

 ■appearance in succession so 

 ■that the egg consists of four 

 |tiers each of eight blasto- 

 Imeres. Eventually, as seg- 

 Imentation proceeds, a blas- 

 $tula is formed of the type 

 ■shown in Fig. 15, A 7. 



The segmentation cavity 



first appears about the time 

 lof the fourth cleavage as a 



small chink. It rapidly 

 ■expands and in the blastula 

 ■figured (Fig. 15, A 7) it is 

 mpi large size. 



In Lepidosiren (Fig. 

 ■15, B) the egg measures 

 ■usually between 6'5 and 7 

 ■mm. in diameter. It is free 

 jp'rom pigment in correlation 

 ■with the fact that it "de- 

 'Jvelops 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- 

 Iwhere it is in large coarse 

 granules. 



The first two furrows 

 ■ Fig. 15, B 2 and 3) are meri- 

 dional and at right angles 



to one another. The third 



set of furrows (Fig. 15, B 4) 

 rare vertical though occa- 



