544 ADAM SEDGWICK. 



R. Rectum. S. 1, 2, 3, &c. The first, second, and third somites, &c. 

 sal. gl. Salivary gland. sep. Septum dividing the lateral portion of the 

 somite from the dorsal, si. g. Slime-gland. S. o. 1. Rudimentary nephridial 

 portion of somite, st. Stomodseum or its lining. T. Tongue, v. e7i. Ventral 

 eudoderm of alimentary canal derived from the cells intermediate between the 

 ectoderm and endoderm when the blastopore was open along its whole length. 

 V. n. Ventral nerve-cord. v. o. Ventral organ, v. s. Ventral sheet of somatic 

 mesoderm. v. sp. Vascular space. w. White matter of central nervous 

 system. 



All the figures are of Peri pat us cap ens is, and drawn with Zeiss's 

 camera, ob. C, oc. 3, unless it is otherwise stated. 



Fig. 1. — Transverse section through a late embryo of Stage a (length 

 '53 mm.), two sections behind the blastopore. The primitive groove is very 

 deep and is hardly to be distinguished from the blastopore. The mesoblastic 

 bands do not extend in front of the hind end of the blastopore. The primi- 

 tive groove was confined to the front end of the streak. The latter extended 

 through eighteen sections. 



Figs. 2 and 3. — Two transverse sections through an embryo of Stage B 



(length '65 mm.) with one somite, still solid, and separate from the front end of 



the mesoblastic band. Primitive streak extended through twenty-one sections. 



Fig. 2. Through the single somite which is present. (The section is 



slightly oblique, passing in front of the somite on the left side.) 

 Fig. 3. Two sections in front of the anterior end of the somite. The 

 endodermal nuclei at the lips of the blastopore were dividing actively. 

 One such is shown at d. )i. 

 Fig. 4. — Section through a late embryo of Stage b (Stage of fig. 25, 

 Pt. I) in front of the mouth. Reduced i. 



Fig. 5, «— /. — A series of sections through an embryo (length 1 mm.) 

 of the same age as the last (Stage b, fig. 25, Pt. I). Reduced i. Four 

 separate somites could be distinctly made out on each side. Thirty sections 

 were obtained through the streak. The groove extends th e whole length of 

 the streak. 



a. Through the mouth. 



i. Between the mouth and anus. The blastopore lips have fused ; their 

 line of fusion is marked by a slight groove (f. bl.). 



c. Through the hind end of the anus. The mesoblastic bands in this 

 region are not yet broken up into somites. 



d. Through the front end of the primitive streak, f our sections behind 

 the last. 



e. Through primitive streak, eight sections behind last. 



f. Through primitive streak, nine sections behind the last. 



