16 ART. 4. N. YATSU: 



relative position of the above planes — whether parallel or at right 

 angles — to each of the preceding cleavage planes could not be 

 determined ; for there is nothing which can serve as an accurate 

 landmark, such, for example, as the polar bodies or the difference 

 in size of the blastomeres. The direction of this cleavage is 

 indicated in PL II., Fig. 28. where the mitiotic figure still lies 

 in the yolk portion. At this stage these blastomeres enclose a 

 spacious segmentation cavity (PL II., Fig. 29). This type of 

 cleavage is considered normal, but there are not wanting many 

 cases out of the usual course. Among them the 12-cell stage is 

 rather frequent, the fourth cleavage having been retarded in four 

 octants. In these instances, however, there result blastulse as 

 perfect as those resulting from the eggs which undergo normal 

 cleavage. 



The fifth cleavage appears in two planes perpendicular to 

 the fourth, thus a lenticular embryo results, composed of two 

 tiers of cells, each tier being made up of 16 blastomeres (PL 

 I., Fig. 9.; PL IL, Fig. 30. 1 ). It should here be stated, 

 however, that the embryos which develop as regularly as this are 

 not many. It is worth while to call attention to an interesting- 

 coincidence in the manner of cleavage up to the 32-cell stage of 

 Lingula with that which goes on in eggs of several species of 

 marine Bryozoa studied by Barrois, Vigelius et al. 



The cleavage processes as just described were not made out 

 by following any one single egg but by observing a number of 

 eggs under cover slips for 12 hours consecutively and by com- 

 paring what was seen in different eggs. For fear that the pressure 

 caused by cover slips might have acted as a mechanical stimulus 



1. This is a section through an egg not exactly at the 32-cell stage but through one 

 in which the cleavage processes have departed a little from the normal course. 



