1 62 Papers from the Marine Biological Laboratory at Tortugas. 



have observed the connecting strand of peripheral protoplasm between two 

 daughter cells. This strand is evidently the result of the unilateral constric- 

 tion of the cell body, and this in turn has been held to be due to the greater 

 amount of peripheral protoplasm on one side than on another (Ziegler, 

 1903). If this be true of the third cleavage as well as of the first, the proto- 

 plasmic layer should be thickest at this stage on the surface adjoining the 

 cleavage cavity, but this is never the case. In my opinion, unilateral con- 

 striction is due in large part to the presence of a more fluid central area, as 

 I shall show in the last section of this paper, and, therefore, when the sub- 

 stance of this central area disappears in the formation of the cleavage cavity 

 and its contents, unilateral constriction during cell division also disappears. 



The escape of the central ground-substance into the cleavage cavity may 

 often be seen to take place by the separation of small globules from the 

 inner ends of the cells, in a manner similar to that in which the so-called 

 " plasmic corpuscles " arise in the blastocoel of Phoronis (Ideka, 1901) and 

 Terebratulina (Conklin, 1902). 



In the 8-cell stage shown in figure 44 the coagulated contents of the 

 cleavage cavity are shown escaping at the vegetal pole, probably owing to 

 shrinkage due to fixation. The 8-cell stage gives rise by meridional cleav- 

 ages to the i6-cell stage (fig. 45). Figures 25 to 27 represent three suc- 

 cessive cleavages of one and the same egg. Up to this stage the cleavages 

 are normally quite regular ;_ sometimes, however, they are more or less 

 irregular, as shown by figures 22 to 24. These irregularities consist mainly 

 in the temporary suppression of the division of the cell-body in one or more 

 of the blastomeres. However, the nuclei in these blastomeres continue to 

 divide by mitosis and the cell bodies subsequently divide (fig. 24) ; such eggs 

 frequently give rise to normal blastulre and gastrulx. I have never observed 

 such irregularities of cleavage in Liiicrgcs as have been seen by Hargitt 

 (1904, 1906) in Pcnihiria, Eiidciidriimi. and Clava. 



In the transition from the 32-cell to the 64-cell stage every nucleus divides 

 by mitosis, as is shown in figure 28. In later stages the cell divisions have 

 not been followed in detail, but I have nowhere seen any evidence of amitosis. 

 The 64-cell stage (fig. 29) gives rise to a stage of about 128 cells (fig. 30), 

 and the latter to a stage of double that number of cells. 



Blastula and gastriila. In a stage of about 500 cells (fig. 31) the 

 cleavage cavity is somewhat eccentric toward the vegetal pole, and the cells 

 at this pole are more rounded and less elongated than elsewhere; these 

 rounded cells are endoderm. In many cases the polar bodies remain at- 

 tached to the egg within the egg-membrane, and in such cases they usually 

 lie at the pole opposite the endoderm pole, as is true of practically all ani- 

 mals. The animal pole of the egg becomes, therefore, the ectodermal pole of 

 the gastrula, the vegetal pole of the egg, the endodermal pole of the gastrula. 



After the 32-cell stage the entire embryo grows larger, apparently 



