300 Dr Allen Thomson on the Vascular System 



serous and mucous layers of the cicatricula (Fig. 7. c c*) ; and 

 that the rudimentary trace of the embryo, which has at this time 

 become evident, is placed in the substance of the uppermost or 

 serous layer (6). According to this observer, and to Baer, the 

 part of this layer which surrounds the primitive trace soon be- 

 comes thicker ; and, on examining this part with care, towards 

 the 18th hour, we observe that a long furrow has been formed 

 in it, in the bottom of which the primitive trace is situated 

 (Fig. 6. dd). About the 20th hour, this furrow is converted in- 

 to a canal, open at the two ends, by the junction of its margins 

 (Fig. 7. dd), (the plica primitiva of Pander, the lamina dor sales 

 of Baer). The canal soon becomes closed at the cephalic or 

 swollen extremity of the primitive trace, at which part it is of 

 a pyriform shape, being wider here than at any other part, 

 (Fig. 8.d'). 



According to Baer and Serres, some time after the canal be- 

 gins to close, a semifluid matter is deposited in it, which, on its 

 acquiring greater consistence, becomes the rudiment of the spinal 

 cord. The pyriform extremity or head is, soon after this, seen 

 to be partially subdivided into three vesicles, which being also 

 filled with a semi-fluid matter, give rise to the rudimentary state 

 of the Encephalon (Fig. 10, df). Rolando, Prevost, and Dumas, 

 on the other hand, suppose that the rudimentary state of the 

 brain and spinal cord is constituted by the primitive trace itself, 

 which, it will be recollected, they affirm exists before incubation 

 has commenced. The view given by Baer of this subject agrees 

 best with my own observations, and I feel the more disposed to 

 adopt it, that the opinion of Prevost and Dumas seems to be 

 influenced by some hypothetical views which they entertain re- 

 specting impregnation. 



As the formation of the spinal canal proceeds, the parts of 

 the serous layer which surround it, especially towards the head, 

 become thicker and more solid ; and, before the 24th hour, we 

 observe on each side of this canal four or five small round 

 opaque bodies. These bodies indicate the first formation of 

 the dorsal vertebrae : in a few hours, several more appear, and 

 those first produced become quadrilateral (Figs. 8. and 10). 

 About the same time, or from the 20th to the 24th hour, the 

 inner layer of the germinal membrane undergoes a further divi- 



