230 



STUDY OF PIG EMBRYOS. 



internal sharp ridge, so that the inner boundary of each neuromere toward the 

 cavity of the fourth ventricle is a small arc of a circle. The cells are elongated 

 and are placed radially to the inner curved surface of the neuromere. A thin but 



distinct layer of ectoglia is present. 



— Sp. 



Lit. 



Scler. 



The light line, which marks the boundary 

 between the adjacent neuromeres, is 

 produced by the comparative absence 

 of nuclei. As to the number of neuro- 

 meres our knowledge is still defective; 

 nor have we yet succeeded in making 

 sure of their exact relation to the nerves 

 of the head, though such a relation evi- 

 dently exists. Thus we find, for exam- 

 ple, that the facial nerve is always con- 

 nected with neuromere 2 of our figure, 

 and the glosso-pharyngeal nerve with 

 neuromere 4. 



Our second section is very near the 

 end of the same series. Owing to the 

 curvature of the posterior end of the 

 body of younger embryos (compare Fig. 

 99; pig 10 mm.), sections taken in the 

 plane which we call transverse strike 

 the lumbar region so as to give longi- 

 tudinal sections of the spinal cord 

 and primitive segments. Figure 135, 

 therefore, shows the cavity of the 

 spinal cord, Sp. c, cut for a very long 

 distance. At the upper and lower ends 

 of the section the dorsal side of the 

 spinal cord is cut, and accordingly we 

 see at these levels sections of the gan- 

 glia, G, on either side of the spinal cord. 

 In the middle of our section the ven- 

 tral portion of the spinal cord is cut, 

 and here, therefore, the ventral roots, 

 V. R, of the nerves are displayed. The segments are clearly marked by the 

 external configuration of the embryo, the ectoderm, Ec, forming an arch over 

 the outside of each segment. Each mesodermic segment shows three distinct 

 parts : next to the ectoderm the broad, epithelioid cutis plate, within which 



Fig. 135. 



-Pig, 6.0 mm. Transverse Series 9, 



Section 519. 



A. is, Intersegmental artery. C«, Cutis plate. Ec, 



Ectoderm. C, Ganglion. muse, Muscle 



plate. Scler, Sclerotome, and. Sp.<; Spinal 



cord. /'. R, Ventral nerve-root. X 5° diams. 



