BY JAS. P. HILL AND C. J. MARTIN. 73 



Fig. 19 (1-9). — Series of transverse sections through the ueuromeric region 

 of the hind-brain. Sections 2, 4, 6 and 8 pass through the 

 neuromeres {n'-n'''^-), while 1, 3, 5, 7 and 9 show the normal 

 thickness of the medullaiy plate in front of and between the 

 neuromeres. ( x 70. ) 



Fig. 20. — Transverse section through the anterior portion of the head 

 process of the primitive streak. Three sections in front of the 

 internal opening of the blastoporic canal. The lumina inch, c.) 

 in the head process represent the last traces of the chorda-cauai. 

 (X 340. j 



Fig. 21.--Tiansverse section through the internal opening of the blastoporic 

 canal [bl, op ) into the cavity of the vesicle. ( x 340.) 



Fig. 22. — Transverse section through the head process of the primitive 

 streak {h. pr.) - 9 sections behind fig. 21. The blastoporic canal 

 is represented by three lumina (6^. c). The head process [h.pr) 

 is distinct from the ectoderm, but shows traces of cellular con- 

 nection with the entoderm. It is more or less continuous with 

 the mesoderm laterally. ( x 340.) 



Fig. 23. — Tninsverse section through the blastopore (W. ). It opens to one 

 side of a longitudinal projection at the front end of the primitive 

 groove Ectoderm, mesoderm and entoderm are fused with each 

 other in the axial line. ( x 340.) 



Plate XIII. 



Fig. 24. — Transverse section through the anterior end of the primitive 

 groove [pr. g.) immediately behind fig. 24. As in that fig. the 

 three germinal layers are continuous with each other axially. 

 (x 340.) 



Fig. 25. — Transverse section through the united Anlagen of the Wolffian 

 duct and tubule in the region of the 7th somite. The Wolffian 

 duct Anlage (vr. d.) is semilunar in form and is connected at the 

 middle of its ventral side with the tubule Anlage {w. t.). The 

 latter possesses a small lumen and is distinct from the somite 

 (m. s.), but connected with th^ lateral mesoderm (mes. /. ). 

 (x 340.) 



Fig. 26. — Transverse section through the Anlagen of the Wolffian duct and 

 tubule some distance behind fig. 25. The Wolffian duct Anlage 

 (w. d.) now exists as a flattened band overlying and distinct from 

 the rounded tubule Anlage. Its outer edges are closely 

 approximated to the ectoderm which immediately over t 



