9, PI, I,, shov; variations of this normal t 

 ore more c : in the type itself, but are easily 



referable to it. . PI. II. shows a section of Fig. 



__z^Z-±* In it one of the two central 



cells is completely cut out of the protoplasm, while, at the 

 nner 



A encl of the cell wall, parti} cutting : - ;, there 



is a little split which in sections nea .vill 



push a short distance to the left but c ight will ex- 



tend clear across, completely cutting out and extending the 



cavity (s.c). The layer of pro: 

 the yolk marke . . is ie central periblast and the cavity 



it is the segmentation cavity. This, however, is 

 not the firs': either since a section in t 



plane a--b in Fig, 4, Pi, I., ow both, I regret 



that I have not been able to find such a section. The outer 

 periblast f o.n . ) never shows the periblastic ridge figured 

 by Wilson ('91) for Sfcrranus. Fig.^" 1 , PI. III., is through 



the plane a--b of Fig, 16, Pi, III,, a normal sixteen-ce] 

 stage, but it will show the state of things in the plane c--d 

 through Fig. 7, PI, II. In this part of the normal blasto- 



of this stage, the central cells ai 

 periblast ( c,p , ) by a lar ntation cavity, which ex- 



tends for a short distance under t 1 cells , in 



