CELL DIVISION IN EGGS OF CREPIDULA. 515 



ectomeres, while A gave rise to a micromere of the first set, much larger than usual, 

 which has divided equally into la 1 and la 2 , the latter being much larger than in 

 the other quadrants; the second division of A was a nearly equal one, the second 

 quartet cell 2A 1 being a macromere. In fig. 66 the axis of pressure was at right 

 angles to the egg axis, B having been crowded under A, and D under C; the first, 

 second and third quartets and their subdivisions are normal, except that some of 

 the cells are slightly displaced. Fig. 67 represents an egg which was flattened in 

 the direction of the egg axis; the three sets of micromeres and their subdivisions 

 are approximately normal; the usual ectodermal cross is present (its cells are 

 stippled) ; the fourth quartet cell (4d) has been formed and is larger than usual. 

 My material showing the effects of pressure does not include stages older than 

 the egg shown in fig. 67, owing probably to the retardation of cleavage in eggs 

 which have been much pressed; in these same experiments many eggs which 

 were pressed less, and are therefore normal, have reached the 64-72-cell stage. 

 In some of the experiments the eggs were under pressure for 18 hrs. and were 

 fixed 4 hrs. after being freed; in others they were pressed 16 hrs. and freed 16 

 hrs., yet in no instance has the cleavage of abnormal eggs advanced beyond the 

 42-cell stage. 



2. Differential and Non-Differential Cleavages. 



Owing to the slow rate at which pressed eggs of Crepidula develop, it is not 

 possible in this case to determine the morphogenetic potency of the various 

 atypical blastomeres which are thus produced. However the cleavage patterns 

 of these pressed eggs show that a meridional cleavage, approximately parallel 

 with the chief axis, divides the egg substance in such manner that every blasto- 

 mere so formed subsequently divides as a normal macromere. If four macro- 

 meres are present, as is the case in normal eggs, each gives rise to three ectomeres; 

 if pressure is applied during the third cleavage, so that five, six, seven or eight 

 such macromeres are formed each still gives rise to three ectomeres; if the pressure 

 is applied after the first set of micromeres are formed so that the macromeres 

 divide equally in the fourth cleavage and by a vertical plane, each of these 

 macromeres in subsequent division gives rise only to micromeres of the second 

 and third sets. If "micromeres" are formed at the third cleavage, which are 

 much larger than usual, as in figs. 53, 59, 62, 63, et al., these cells do not become 

 ectomeres in their entirety but they cut off protoplasmic ectomeres at the animal 

 pole from yolk-containing entomeres at the lower pole. If the third cleavage is 

 rendered equatorial, each of the cells of the animal pole gives rise to three sets 

 of ectomeres, but those at the vegetal pole produce no ectomeres. If the cleavage 

 plane is oblique, between meridional and equatorial, the cells so formed may give 

 rise to a varying number of ectomeres, and in general only macromeres which 

 reach the animal pole produce three ectomeres, while the farther the macromeres 

 are removed from that pole the smaller the number of ectomeres which they 

 produce. 



These facts are important with reference to one of the greatest problems of 



