THE DEVELOPMENT OF HYDRA. 269 



(Fig. 19). The cells within the cleavage cavity also divide very 

 rapidly (PL X., Fig. 20), and the embryo becomes a solid spher- 

 ical mass of cells with the cleavage cavity entirely obliterated. 



The formation of the endoderm begins uniformily at the differ- 

 ent poles of the blastula. Its origin is multipolar as Brauer states. 

 According to Brauer, during the formation of the endoderm some 

 of the cells which divide radially and remain within the wall of 

 the blastula force the primitive ectodermal cells, which have a 

 narrow base, from the periphery into the cleavage cavity before 

 they divide, and thus an entire cell of the primitive ectoderm 

 becomes an endodermal cell. The species studied showed a num- 

 ber of cells dividing radially in the periphery during the formation 

 of the endoderm, but I was unable to find any indication that 

 entire cells were forced from the periphery into the cleavage 

 cavity. 



According to Kerschner (5) and Korotneff (6), the endoderm 

 is formed by the inwandering of cells from the vegetative pole of 

 the egg. 



When the endoderm is completely formed division ceases and 

 the endodermal cells with their abundance of yolk can readily be 

 distinguished from the cells of the outer layer or ectoderm. 



Egg Membranes. 

 The outer and inner egg membranes are formed from the ecto- 

 dermal cells. The outer membrane begins as an outgrowth from 

 the different cells of the ectoderm. A very small portion or 

 nearly all of an ectodermal cell may take part in this process (PL 

 X., Figs. 20-22). These outgrowths in the early formation of 

 the membrane remain continuous with the cell from which they 

 originate, and are often nearly as large as the body of the cell 

 itself (Fig. 23, o). The outer protoplasmic ends of these projec- 

 tions assume various shapes (Fig. 25). The thin elastic wall 

 surrounding these outgrowths, which is a continuation of the wall 

 of the ectodermal cell, often becomes very delicate, breaks through 

 and allows the cytoplasm of the cell to flow out, thus causing the 

 cell to collapse. The different outgrowths now fuse at their basal 

 ends and form a continuous membrane around the developing 

 embryo (Fig. 25). This membrane becomes very tough and has 



