JEXNIXGS : DEVELOPMENT OF ASPLAXCHNA HEREICKII. 105 



46) are at opposite sides of the nucleus, the hue joining them being 

 parallel to the latei'al axis of the embrj-o. 



In the egg seen in Figui-e 53 (Plate 7), the fifth division is finished 

 in all the cells of the egg, (as shown by the study of the other quad- 

 rants, which are not figured,) so that this is a later stage than that 

 shown in Figures 43-46. The asters in d^-^ have taken an oblique 

 position. 



In the egg of which Figures 48 (Plate 6) and 54 (Plate 7) are repre- 

 sentations, the nuclei in the recently formed cells of quadrants A, B, 

 and Chave enlarged, and the spindle is completely formed in d^-^ (being 

 dorso-ventral in position) . Both of these facts show that the egg is older 

 than the one shown in Figure 53. In this egg (Fig. 48) we find that a 

 spindle is present in the entoderm cell c?®-\ occupying nearly the position 

 foreshadowed by the position of the asters in Figure 42, and almost 

 exactly the same position as the spindle at the foregoing division (Fig. 35). 



The egg seen in Figure 49 is still older, as shown by the presence 

 of spindles in d^-^ and d^-'' (seen endwise), and the advanced condition of 

 cleavage in d^-^. Here d^-^ is just dividing, forming d''-^ and the second 

 small cell, d"''^. 



Figure 50 is older than Figure 49, since d^-^, d^-'', and d^'^ have divided. 

 In this egg we find that d^'^ has been separated into two cells, d''-^ and 

 d~-^, and there is a second small vesicle, d'-', in the position where it 

 was seen in the process of formation in Figure 49. 



The ahove is sufficient to illustrate the method of work ; the rest of 

 the account might be analyzed in the same way. It is important to 

 remember, however, that the description is not based merely upon the 

 cases figured. Thus, for the processes just analyzed, more than thirty 

 eggs, showing various phases of the changes occurring, were studied, 

 while only eleven different eggs ai'e represented in the figures of these 

 stages. 



The foregoing work was done in the winters of 1894-95 and 189.5-96, 

 in the Zoological Laboratory of the Museum of Comparative Zoology at 

 Harvard University, It gives me pleasure to acknowledge my great 

 indebtedness to the Director of the Laboratory, Professor E. L. Mark, 

 for advice and assistance which have been of the greatest value to me 

 throughout my work. 



