DEVELOPMENT OP PERIPATUS NOV^-ZEALANDIiE. 261 



Fig. 17a is a somewhat oblique section through the anterior end of the 

 ovum. It passes through the posterior tip of the embryo {post. Em.), 

 which is distinct from the ventral wall of the anterior end, being sur- 

 rounded by a complete layer of ectoderm. Owing to the obliquity of 

 the section the right prseoral lobe is considerably larger than the left. 

 Fig. n b is through the middle of the ovum, where the anterior and 

 posterior ventral surfaces of the embryo are only separated from one 

 another by a single protoplasmic septum {Sep.) 

 Fig. 17 c is through the hind part of the egg, behind the region of the 

 septum, where the anterior and posterior portions of the embryo are 

 in free communication with one another. 

 Figs. 18« — c. — Three transverse sections through an ovum, rather older 

 than that figured in Figs. 11 a— c. Ec. Ectoderm, lies. Mesoblast. p. o. I. 

 Prseoral lobes, /j.j/. Remains of peripheral yolk. /S". Somites. »?/. Stomodaeal 

 invagination. V. m. Vitelline membrane. T. Yolk. Oc. 3, obj. A. 



Fig. 18a passes through the anterior end of the embryo, in tiie region of 



the prseoral lobes. The ectoderm has begun to invaginate in tlie middle 



ventral line to form the stomodseum, and the somites of the praeoral 



lobe segment are present (S. i.). The space between the embryo and 



the vitelline membrane is occupied by a large number of the small 



round bodies, wliich are also present among the tissues of the embryo. 



Fig. 18 i passes through ihe middle of the ovum. The ventral surfaces 



of the anterior and posterior regions of the body are completely 



separate from one another. The somites of the trunk are beginning 



to appear {S.) 



Fig. 18 c passes through the hind of the embryo, where the anterior and 



posterior portions of the embryo are continuous. 



Figs. 19 a— (^.— Four rather oblique transverse sections through an ovu'n, 



in which the embryo is still doubled on itself, though rather less so than in 



the ovum from which Figs. 18£«— c were taken, and the space between the 



apposed ventral surfaces is greater. In this ovum the endoderm is first 



definitely differentiated as a distinct layer. A few small round bodies are 



still present between the vitelline membrane and the ectoderm, but most of 



Ihem have by this time been absorbed. Cer. Com. Cerebral commissure. 



Ec. Ectoderm. End. Endoderm. M. Mouth. Mes. Mesoderm, p. o. I. 



Prseoral lobes, post. Em. Posterior tip of the embryo. S. Somite. V. m. 



Vitelline membrane. Oc. 3, obj. A. 



Fig. 19a passes through the anterior end of the embryo in front of the 



mouth, in the region of the cerebral commissure. 

 Fig. 19 b passes through the mouth of the embryo. The posterior tip 

 is shown lying on the ventral side of the ovum, separated by a con- 

 siderable space from the mouth. 

 Fig. 19 c is from a section posterior to the above. It passes through 

 the anterior part of the embryo behind the mouth, on one side passing 



U 



