262 LILIAN SHELDON. 



through the posterior end of the praeoral 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. IQd 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 « — c. — Three transverse sections through the embryo, which is 

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

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

 )i. d. Rudiment of nephridial duct. n. s. Ventral nerve-cord. FA. Pharynx 

 S. Somite. Sp. Splanchnic wall of somite. So. Somatic wall of somite. 

 Oc. 4, obj. A. 



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



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

 Fig. 216 passes through the appendage of the left side, and shows the 

 somite dividing into 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 into appendages. 

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

 posterior to that shown in Fig. 21c, but it is more highly magnified. End. 

 Endoderm. pr. ff. Primitive groove, pr.st. Primitive streak. Oc. 2, obj. D. 



Fig. 23. — Surface view of a segmenting ovum, to show the yolk segmen- 

 tation. Ck. Chorion. F. S. Yolk segments. 



Fig. 2i. — Surface view of a segmenting ovum. The protoplasmic segments 

 are seen lying on the surface of the egg. The yolk segmentation is not seen, 

 owing to the surface of the egg having been slightly disintegrated by the 

 preserving reagents. Tiie chorion and vitelline membrane have been removed. 



Fig. 25. — An embryo, in which all the peripheral nutritive layer has been 

 absorbed, viewed from the side ; showing the large prseoral lobe with the 

 antenna budding out from it, and the lateral ridge with five distinctly formed 

 appendages {dp.). 



Fig. 26. — The same embryo seen from the ventral side, and showing the 

 mouth and anus and primitive groove, in addition to the structures seen in 

 the last figure. These two drawings were from an embryo preserved in picric 

 acid. an. Anus. Ap. Appendages. At. Antennse. M. Mouth, p. o. I. 

 Prseoral lobe. pr. gr. Primitive groove. 



