THE DEVELOPMENT OF THE FOWL'S SKULL. L37 



of the head; and the dorsal laminse, thus produced, extending 

 forwards and backwards, like parapets, upon each side of the 

 primitive groove, lay the foundations of the lateral walls, not 

 only of the skull, but of the spinal column. 



Very early, however, the boundary line between skull and 

 spinal column is laid down, by the appearance in the substance 

 of the bases of the dorsal lamina) and the adjacent middle layer 

 of the blastoderm, of the first pair of those quadrate masses 

 of condensed tissue, the jproto-vertebrte (" Urwirbel ' : of the 

 German writers), which are the foundations, not only of the 

 bodies of the vertebra?, but of the spinal muscles and ganglia. 

 The proto-vertebrse increase in number from before backwards ; 

 and, at length, extend through the whole range of the spinal 

 column, while none ever make their appearance in the region 

 which will be converted into the skull. 



The edges of the dorsal laminse now unite, the coalescence 

 taking place first in the middle cephalic region, and extending 

 thence backwards and forwards ; at the same time, the cephalic 

 canal becomes separated into three distinct dilatations, or cere- 

 bral vesicles, of which the anterior is by far the most marked 

 (Fig. 57, A, I, II, III). 



The rudimentary cranial cavity next becomes bent upon 

 itself in such a manner, that the longitudinal axis of the first 

 cerebral vesicle takes a direction at right angles to the axis of 

 the third, and of the spinal canal generally. In consequence of 

 this change, the middle cerebral vesicle occupies the summit 

 of the angulation, and becomes the most anterior point of the 

 whole body (Fig. 57, C, D). 



The bend thus produced is the cranial flexure. It results 

 in the division of the floor of the cranial cavity into two parts, 

 an anterior and a posterior, which are at right angles to one 

 another (Fig. 57, C, D, E). Hitherto, no trace of the noto- 

 chord has been observed in the anterior division, that structure 

 ending in a point behind the flexure (Fig. 57, D, E, li). 



As development proceeds, the anterior cerebral vesicle be- 

 comes divided into two portions, — an anterior, the vesicle of the 

 cerebral hemispheres (J") ; and a posterior, the vesicle of the 

 third ventricle (I ). In the upper wall of the vesicle of the third 



