ON THE DEVELOPMENT OF LINGULA ANATINA. 15 



The second cleavage plane appears at right angles to the 

 first (PL L, Fig. 4.). When the cleavage is finished there is 

 sometimes found a space at the centre of four blastomeres, but this 

 is by no means always the case. There are some cases in which 

 each quadrant elongates peripherally so that the egg assumes as 

 a whole the shape of a cross or four leaved clover. From this 

 cleavage onward all imaginable irregularities creep into the manner 

 of cleavage. In some eggs the second cleavage is finished in one 

 blastomere while the other remains without constriction (PI. L, 

 Fig. 3.). In the cleavage of the egg of Clstella Shipley ('83) 

 gives a figure (Taf. XL., Fig. 22.) of just the same case. The 

 retardation of cleavage in a blastomere does not seem to depend 

 upon the difference in size of the latter, but upon some unknown 

 causes. It should here be noted that from the second cleavage 

 on, the chromosomes appear as rods unlike those in the first 

 (PI. II., Figs. 28. 30.). 



The third cleavage cuts perpendicularly to the preceding two, 

 thus giving rise to eight blastomeres (PI. I., Fig. 5.). A section 

 at this stage is shown in PL IL, Fig. 27 where we can distinctly 

 see the segmentation cavity at the centre. The dissolving of yolk- 

 granules proceeds in each blastomere from the outer surface inward, 

 so that only the yolk-mass remains near the surface of the seg- 

 mentation cavity. This behavior of the yolk can be noticed up 

 to young embryos. Toward the end of the stage each blastomere 

 becomes slightly compressed, so that the egg as a whole is 

 flattened in one direction as we can readily see on comparing 

 PL I., Fig. 6 and 7. In these figures the indication of the 

 cleavage plane next to appear is noticeable. 



The fourth cleavage takes place in two parallel planes which 

 pass through the centre of each octant (PL I., Fig. 8.). The 



