54 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



the endoderm remains exposed on the surface. A cavity now 

 appears in the endodermal mass, and opens externally where the 

 endoderm is not covered by the ectoderm. The opening is the blasto- 

 pore (fig. 22 A, Bpr). With the growth of the embryo, the blastopore 

 lengthens to an elongate slit on the ventral surface (B). The first 

 observations on the development of Peripatopsis capensis were made 

 by Balfour ( 1883), who believed that the mesoderm arises in the form 

 of paired coelomic pouches along the edges of the elongate blastopore 

 where the ectoderm and endoderm are confluent. From the subse- 

 quent work of Sedgwick, however, it appears that the mesoderm in 

 P. capensis is generated from an opaque area of the blastoderm 

 situated behind the posterior end of the blastopore (A, B, C, MsT). 

 From this area, or "primitive streak," there takes place an internal 

 proliferation of cells, which, migrating forward in each side of the 

 embryo, produce two ventrolateral mesoderm bands along the margins 

 of the blastopore (B). The bands then break up into sections that 

 mark the primitive somites of the embryo, and later are excavated by 

 the coelomic cavities (G, Coel). The elongate blastopore finally closes 

 by the fusion of its lips, except at the two ends, which become the 

 primary mouth and the primary anus (D, Mth, An). 



The development of the endoderm of Eoperipatus zveldoni, as 

 described by Evans (1902), is again different from that of Peri- 

 patopsis capensis. "The endodermal elements," Evans says, "are 

 derived from the lips of the blastopore and travel inward along the 

 outer layers of the yolk, which is at first devoid of nuclei." Here, 

 evidently, is a process suggesting invagination. On the surface of 

 the yolk the endoderm cells form a complete investing layer, but later 

 some of them invade the yolk, probably bringing about its partial 

 digestion, and then again most of these cells return to the surface, 

 where they reconstruct a permanent endodermal sac containing the 

 yolk (fig. 22 H, End). A few endodermal cells, however, remain 

 within the yolk. The mesoderm of Eoperipatus zveldoni, according 

 to Evans, is formed in the same way as described by Sedgwick for 

 Peripatopsis capensis, that is, from a proliferating area of the blasto- 

 derm situated immediately behind the blastopore (E, MsT). 



Considering the various processes by which the organization of 

 the onychophoron is accomplished in the embryo, it would appear 

 that the manner of development has little significance. In extreme 

 cases the assembling of the germ layers seems to be almost haphazard. 

 Sheldon (1888) observes of Peripatoides novae-zealandiae that the 

 embryo might be said to be formed "by a process of crystallizing out 

 in situ from a mass of yolk, which is a protoplasmic reticulum con- 



