DEVELOPMENT OF PEUIPATUS NOV^-ZEALANDL^. 235 



Fig. 15 b. Section through the anterior part of the embryonic region, 



showing the embryo surrounded by the peripheral yolk layer. 

 Fig. 15 c. Section through the middle of the embryonic region, showing 

 the embryo surrounded by the peripheral yolk and enclosed in the 

 ectoderm, on one point of which is the proliferating mass of nuclei. 

 The small round bodies are shaded very dark. 

 Fig. 15 d. Section through the posterior end of the embryo, shortly an- 

 terior to its termination and behind the region of the proliferating 

 ridge. 

 Fig. 16. — Shows a group of the small round bodies of the peripheral yolk 

 layer from the embryo shown in Figs. 18a — c, highly magnified. They are 

 vacuolated. Reichert's ^ oil immersion. 



Figs. 17« — c. — Three sections through an embryo, somewhat older than that 

 from which Figs. 15 a — d were drawn. Ec. Ectoderm. Mes. Mesoblast. 

 p. o. I. Prseoral lobes, post. Em. Posterior tip of the embryo. Sep. Septum. 

 V. m. Vitelline membrane. Oc. 4, obj. A. 



Fig. 17 a 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. 17 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 embrjo are 

 in free communication with one another. 

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

 than that figured in Figs. 17 a — c. Ec. Ectoderm. Mes. Mesoblast. p. o. I. 

 Prseoral lobes, p. y. Remains of peripheral yolk. S. Somites. St. Stomodseal 

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



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

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

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

 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, which are also present among the tissues of the embryo. 

 Fig. \%b passes through the 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. 



