GEKMINAL LOCALIZATION IN THE EGG OF CEREBRATULUS. 21 



able, however, that nearly perfect larvœ were derived from both 

 the animal and the vegetal quartet«. The cases are unfortunately 

 not numerous enough to show^ how large a percentage of such 

 larvae is really formed. This may be attributed to the variation 

 of the level of the third cleavage. As a matter of fact in the 

 egg of C. lacieus the cleavage usually takes place a little below 

 the equator, but sometimes the plane rises even a little above it. 

 Or this may be due to the variation of the extentioii of the 

 entodermic stuff in the egg. 



Series R. Pressure Experiment followed by 

 Cutting at the 4- cell Stage. ' 



At the 2-cell stage several eggs 

 were compressed under a large cover 

 glass. Of the eggs those upon which 

 pressure happened to be properly 

 applied, cleaved along the equator, 

 giving rise to four blastomeres ar- 

 ranged vertically (Yatsu '10 p. 7). 

 Then the second cleavage was com- 

 pleted the pressure was removed and by 

 means of a knife the upper two cells 

 (2A) were cut apart from the low^r 

 (2V) as is indicated in the diagram 

 (Fig. 19). In this way I succeeded in bisecting the egg exactly 

 along the equator.^ The operation yielded ten pilidia : 



Fig. 19. 



Diagram showing compress- 

 ed egg and plane of section 

 in Series E. 



5 A preliminary report of this experiment has appeared in the Proceedings of tiie 

 Society for experimental Biology and Medicine (1907). 



6 Before I hit upon this method I had tried in va'n Driksch's metliod of sliifting 

 the tliird cleavage in rearing the egg in diluted sea-water. 



