228 W. L. SCLATER. 



EXPLANATION OF PLATE XXI 



Illustrating Mr. W. L. Sclater's Memoir, ''On the Early 

 Stages of the Development of the South American Species 

 of Peripatus." 



Complete List of Reference Letters. 



am. So-called amnion, c. Cuticular layer, ck. Folded inner edge of the 

 uterine epithelium, e. Embryo proper, ec. Ectoderm, en. Endoderm, ,/. 

 Cells of doubtful function, probably nourishing, mes. Mesenteron. n. Nuclei 

 of vacuolated epithelium, n. c. Nerve-cord. o. Artificial cavity due to re- 

 agents, p. b. Polar body. pg. Pigment, pi. Vesicle swelling (placenta). 

 pr. Primitive streak, sp.c. Supporting cells, u.e. Uterine epithelium, ut. 

 Uterus. V. Vesicle, v.e. Vacuolated epithelium, v.w. Vesicle wall. z. 

 Embryonic swelling (embryonic placenta) whence the stalk arises. 



All the figures, except Figs. 1 and 11 to 13, are drawn with Zeiss's camera 

 obj. D, oc. 2), but Figs. 16 and 17 have been reduced to one half the original 

 size. 



Fig. 1. — Longitudinal section through the upper part of the uterus, showing 

 a young embryo and the curious histological structure of the uterus. Drawn 

 with Zeiss's camera. (Obj. A, oc. 2.) 



Fig. 2. — Youngest embryo met with, measuring *04 mm. in diameter. It 

 seems to consist of eight nuclei with surrounding protoplasm. 



Fig. 3. — Embryo segmenting rather more advanced with polar bodies. 



Fig. 4. — Embryo with commencing central cavity. 



Fig. 5. — Segmenting embryo drawn with the surrounding uterus to show 

 the structure of the uterus and its relation to the embryo. 



Fig. 6. — Young blastosphere stage before gastrulation and after the com- 

 pletion of the segmentation. 



Figs. 7a and 7b. — Two sections of an embryo in the pseudogastrula stage. 

 Fig. 7a shows the condition of the embryo at one end beyond the infiuence of 

 the invagination ; Fig. 7b is a transverse section through the gastrula and its 

 blastopore. 



Fig. 8. — Youngest embryo found in a vesicle. The wall of the vesicle is 

 much broken, only a very small part being represented in the figure. 



Figs. 9a and 9b. — Two sections through a vesicle and embryo of moderate 

 size. The embryo is still sessile. Fig. 9a represents a section through the 



