744 PROCEEDINGS OF THE ACADEMY OF [Oct., 



neither at this stage nor at any of the later ones have I been able to 

 determine the presence of centrosomes. 



The double egg-membrane, which we saw had become separated 

 from the surface of the egg, is still present and continues to include 

 the embryo until the larval stage is a week old or more. It is a com- 

 plete protection for the egg against any bacterial organisms. 



In the matter of the cleavage I have spent much time in the en- 

 deavor to work out the cell-lineage, but have been unable to do so. 

 The egg is spherical ; the second polar body varies in its position with 

 regard to the axis of the first cleavage spindle; the yolk appears to 

 be rather uniformly distributed near the periphery. So there is no 

 means of orientation of the uncleaved egg. The cleavage is total and 

 adequal (figs. 17-21). The two blastomeres of the 2-ccll stage are 

 almost equal in volume, but one is always slightly larger than the 

 other (fig. 17). In the 4-cell stage (fig. 18) there are two larger blasto- 

 meres and two smaller ones, but the difference is very slight. The axes 

 of the spindles in the 2-cell stage may be parallel or not, and the cells 

 may divide simultaneously or successively. In the 4-cell stage it is 

 most frequent that a line connecting the two smaller blastomeres is 

 perpendicular to one joining the two larger, in such a way that all four 

 blastomeres do not lie in one plane; they apparently never arrange 

 themselves in the T-shape so characteristic of Ascaris. Then the 

 blastomeres are not spherical, but so moulded by mutual contact that 

 it is exceedingly difficult to determine their relative volumes. Again, 

 the blastomeres of the 4-cell stage may divide simultaneously or not. 

 For these reasons I have found it impossible to work out the cell- 

 lineage, without, however, wishing to imply that it may not be per- 

 fectly determinate. 



A coeloblastula (fig. 20) is formed at an early cleavage, sometimes 

 as early as six blastomeres, almost always at the 8-cell stage (fig. 19). 

 At the 16-cell stage (fig. 21) the cleavage-cavity is large. On optical 

 sections one pole of the blastula shows cells somewhat larger than 

 those of the opposite pole. 



While there are but a relatively small number of blastomeres, an 

 invagination of the larger-celled wall of the blastula commences and 

 leads to the establishment of a typical invagination gastrula (figs. 22- 

 27). All the lining (mesentoderm) of the gastroccel is not formed b}^ 

 an inpushing of the wall of the blastula, but it appears rather that only 

 a small number of cells are invaginated, and that by the cell division 

 of these the mesentoderm increases in amount. The orifice of the 

 invagination, the blastopore is very narrow, and in a position that 



