236 LILIAN SHELDON. 



.Figs. 19 a — d. — Four rather oblique transverse sections through an ovum, 

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

 the ovum from which Figs. 18 a — 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 

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

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

 Preeoral lobes, post. Bm. Posterior tip of the embryo. S. Somite. V. m. 

 Vitelline membrane. Oc. 3, obj. A. 



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



mouth, in the region of the cerebral commissure. 

 Fig. 193 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 

 through the posterior end of the prseoral lobe ; and through the pos- 

 terior part of the embryo in the region behind that where the somites 

 are present, the mesoblast (Mes.) being solid. 

 Fig. 19 d passes through the posterior end of the ovum, in the region 

 where the anterior and posterior portions of the embryo are con- 

 tinuous with one another. 



Fig. 20. — Transverse section through an embryo which has become almost 

 completely straightened out. The somatic wall (So.) of the somites is 

 thickened, as also is the ectoderm lying over it. End. Endoderm. n. s. 

 Rudiments of ventral nerve- cords. S. Somite. Sp. Splanchnic wall of 

 somite. So. Somatic wall of somite. Oc. 4, obj. A. 



Figs. 21 a — c. — Three transverse sections through the embryo, which is 

 shown in surface view in Figs. 25 and 26. an. Anus. Cer. Com. Cerebral 

 commissure. End. Endoderm. br. Brain. L. S. Leg portion of the somite. 

 n. d. Rudiment of nephridial duct. n. s. Ventral nerve-cord. Ph. Pharynx 

 S. Somite. *S^. Splanchnic wall of somite. So. Somatic wall of somite. 

 Oc. 4, obj. A. 



Fig. 21a is taken just in front of the mouth, in the region of the cerebral 



commissure. The pharynx is seen in communication with the gut. 

 Fig. 21 b passes through the appendage of the left side, and shows the 

 somite dividing iuto a leg and a body portion, and also the ventral 

 outgrowth which will form the nephridial duct. 

 Fig. 21c passes through the anus, and the lateral ridge behind the region, 

 where it is divided iuto appendages. 

 Fig. 22. — Shows the primitive streak, and is taken from a section a little 



