114 THE DEVELOPMENT OF 



That we are justified in making these assumptions is 

 shown by several things. First, Haeckel naturally took 

 the bodies inside the lobes of the egg for nuclei alone, as 

 at that time the structure of the cells was less understood 

 than at present, and from what was known of other eggs, 

 that seemed the only way to regard them. A comparison 

 of HaeckePs figures with those of other students of Deca- 

 pod segmentation shows that this explanation accords well 

 with what is known of other forms. Thus Mayer ('77) 

 describes, in the segmenting egg of Eupagurus, nuclei sur- 

 rounded with a layer of protoplasm extending out, amoeboid 

 fashion, with the surrounding yolk. These must be re- 

 garded here, as in Crangon, as true cells, and their origin 

 from the original nucleus and protoplasm must have been 

 by segmentation in the centre of the egg. As in Crangon, 

 they migrate to the surface and form a blastoderm envelop- 

 ing an unsegmented mass of yolk. Faxon ('79), though 

 he cut no sections, clearly shows that in Palsemouetes the 

 same is the case. His figures 1 and 2 represent the nucleus 

 surrounded in the same way with its protoplasm. Ishikawa 

 ('85) apparently obtains the same result in Atyephyra, 

 judging from his plates. His figures 35 and 36 are espec- 

 ially interesting in this connection, for they appear to sub- 

 stantiate the view here maintained, and when taken in 

 connection with figures 38 and 39 clearly show that there 

 is a migration of cells to the surface. 



The extremely scanty observations on the segmentation 

 of Limulus by Osborn ('85), and by Brooks and Bruce 

 ('85) do not allow us to arrive at any very definite con- 

 clusions as to the character of the division, but the fact that, 

 according to the last-mentioned authors, at the close of 

 segmentation the entire yolk "consists of a uniform mass 

 of large spherical yolk cells, each with its nucleus," would 

 seem to indicate that here the segmentation is not "super- 

 ficial ;" while on the other hand, there is nothing in either 



