196 ONYCHOPHORA. 



syncytium containing nuclei and. rich in vacuoles. This voluminous 

 nucleated mass must no doubt be regarded as corresponding to the 

 yolk with its nuclei found in P. novae-zealandiae. In the latter 

 form, the nucleated yolk forms part of the boundary of the arch- 

 enteric cavity. In both forms the blastopore lengthens (Fig. 99 A), 

 and is constricted in the middle of its length, where its edges become 

 approximated and fused ; thus the original single blastopore becomes 

 divided into two apertures (Fig. 99 A-C). During this process, the 

 vacuolated entodermal syncytium becomes arranged into a regular 

 epithelium which, where the blastopore is still patent, passes over 

 into the ectoderm, but in the region of the closed blastopore forms 

 a tube that is said at first to be connected with the mesoderm-bands 

 lying in that region, but to become isolated later, thenceforth form- 

 ing a distinct entodermal tube. 



In P. novae-zealandiae, in consequence of the large amount of yolk present, 

 this process is somewhat different. The entoderm-cells are here said to become 

 arranged at the periphery of the yolk into an epithelium which thus surrounds 

 the yolk. The latter would then he gradually absorbed during the further 

 development of the intestine. The mouth and anus form as in P. capensis 

 (Sheldon). 



The two apertures derived from the elongation and constriction 

 of the blastopore (Fig. 99 D) are the primitive mouth and anus. 

 They do not, however, persist as those organs in the adult, owing 

 to the appearance of a depression of the ectoderm at each of the 

 openings, so that the point of union between the ectoderm and 

 the entoderm is shifted inward, and an ectodermal stomodaeum 

 and proctodaeum are formed. 



The changes of form undergone by the embryo have their influence 

 on the rudiment of the intestine. As a consequence of the curvature 

 of the embryo, the entoderm extends anteriorly and posteriorly above 

 the mouth and the anus (Fig. 98 A). The anterior wall of the 

 stomodaeum thus runs forward. During the further development 

 of the embryo, however, the course changes. When the mouth is 

 shifted more to the anterior end, the anterior entodermal sac 

 degenerates, and the stomodaeum now appears directed posteriorly 

 (Fig. 98 B). The dorsal wall of the anterior portion of the 

 intestine up to this point was closely apposed to the body-wall 

 (Fig. 98 A and B), but the latter now separates from the gut and 

 forms the swollen anterior end of the embryo (Fig. 98 C). It is 

 followed in this course by a diverticulum of the entoderm, while 

 the stomodaeum retains its former position. This diverticulum is 



