No. 6.] DEVELOPMENT OF PARYPHA CROC E A. 



305 



ec 



o 



ci 



FIG. i x 560. Early stage in the formation of 

 the ectoderm of the embryo ; ec, ectoderm ; en, 

 endoderm ; w, nuclei ; w 1 , nuclei in the process 

 of division. 



spindle-shaped, with one end broader than the other. Both the 



form of the cells and the position of the nuclei indicate that 



they have been formed from a 



condition like that in Fig. 2, 



and not by further delamina- 



tion of the outside layer alone. 



At this stage the larva is 



made up of a solid mass of 



irregular cells with spherical 



nuclei surrounded by a single 



layer of much elongated cells. 



No segmentation cavity is 



formed. The origin of the 



germinal layers agrees, there- 

 fore, quite closely with that 



described by Hickson ('93) 



under E. c., p. 52: "A ster- 



rula is formed by precocious 



delamination. No segmenta- 

 tion cavity is formed, and segmentation is at first incomplete." 



Parypha is not mentioned by Hickson under this class, and 



Tubularia, the form most like Parypha 

 in its general mode of development, he 

 includes under another head. Dr. Har- 

 gitt informs me that nothing equivalent 

 to true delamination or invagination oc- 

 curs in Pennaria. It would, therefore, 

 seem that no one, two, or even three 

 laws of cleavage are sufficient to explain 

 the varied conditions to be found in the 

 segmentation of the hydroid egg. 



The embryo now appears concave 

 upon the side next the manubrium, but 

 this is probably due to pressure and 

 not to any intrinsic cause. After the 

 formation of the ectoderm the two 



layers of the embryo evaginate at seven points so that a 



section through the region of the process appears star-shaped, 



FIG. 2 x 560. Later stage in the 

 formation of the ectoderm ; ec, 

 ectoderm ; en, endoderm ; a, nu- 

 clei of absorbed cells. 



