kofoid: DEVELOPMENT OF UMAX. 47 



have no limiting membrane ; they are, however, devoid of the granular 

 structure of the surrounding protoplasm, and are the centres about 

 which the radiations constituting the asters are arranged. 



The position of the astroccels with reference to the nuclei is worthy of 

 note. They are removed some distance from the nuclei toward the 

 animal pole of the cells in which they lie. A comparison of this figure 

 with that of a later stage shown in Figure 5 indicates that the astroccels 

 are migrating toward a region where later the nuclei are found. It must 

 seem therefore from the conditions in Figure 14 that the nuclei are pre- 

 ceded in this migration by the astroccels. This recalls the shifting of 

 male and female pronuclei attributed to the astroccels by Conkhn ('94) 

 in Crepidula. 



In the living egg of this stage, when the cells have reached a perfectly 

 spherical shape, each blastomere seems to be entirely independent of 

 the other, and not the least trace of any contact or connecting proto- 

 plasm can be detected between them. Each has a definite, unbroken 

 contour, and in most cases there is an appreciable space between them, 

 which shows no differentiation from the surrounding albumen. In the 

 egg shown in Figure 14 the separation is not so great as it apparently is 

 in the living egg. It is an interesting phenomenon, and raises the ques- 

 tion as to the existence of any actual protoplasmic connection between 

 the blastomeres in the stage following constriction. It is impossible to 

 answer the question satisfactorily from observation of the living egg, 

 for there is the possibility of the existence of a thin sheet of pi-otoplasm 

 which, on account of its transparency, thinness, and optical resemblance 

 to the surrounding albumen, cannot be detected. The egg shown in 

 Figure 14 was shelled by the process described in the preceding pages 

 and washed free from the albumen by normal salt solution, transferred 

 in capillary tubes a number of times in the process of preparation, and, 

 after mounting in balsam, was rolled over in various directions repeatedly 

 ivithout a separation of the two blastomeres. The two cells have each of 

 them a definite and sharp outline at all planes of focusing, and even 

 under high powers of the microscope no deeply stained granular bridge 

 of protoplasm can be detected between them. It is only by very care- 

 ful focusing that the rather vague, transparent, unstained connection 

 between the cells can be seen. So far, then, as this preparation goes, it 

 shows that there is a physical band of connection between the two 

 blastomeres in this stage of greatest separation. The nature of this 

 connection is problematical. It may be the Schleimschicht of Warneck 

 ('50), or it may be a continuation of the " differentiated superficial 



