528 



VERTEBRAL COLUMN. 



Fig. 260. 



Origin of the train upon the spinal cord, magnified 8 

 diameters. 



the other, the wider part being destined for the head of the embryo. 

 On each side of the primitive groove two oval areas of cells emerge : 

 they are the dorsal laminae. They rise up to cover in the primitive 

 groove, so as to convert it into a tube, with three bead-like swellings at 

 its wider end, the elements of the prosencephalon, mesencephalon, and 



epencephalon, Fig. 260. The ex- 

 planation of this and the preced- 

 ing figure have already been given 

 on p. 293. On the internal part 

 of the lamina nervous matter be- 

 gins to form, the rudiment of the 

 cerebro-spinal axis. In the bot- 

 tom of the groove is the trace, 

 chorda dorsalis. The groove it- 

 self, converted into a tube, con- 

 stitutes the central canal of that 

 axis, its completion into the tu- 

 bular shape occurring first in the 

 middle, and then up and down. 

 The form of the lateral masses va- 

 ries as development goes on. 

 A line of cells running lengthwise in the primitive groove is the origin 

 Chorda dorsa- of the chorda dorsalis, on which the rudiments of the verte- 

 and~^vascuiar kral c l umn appear. In the amphioxus and myxenoid fishes 

 system. development in this direction stops at this point, the chorda 



dorsalis being the permanent structure. The vertebrse now emerge un- 

 der the aspect of square plates, and the dorsal laminse, prolonging them- 

 selves outward and downward, as it were, by an offshoot, produce the 

 ventral laminse, which close in the abdominal walls, and so form the 

 boundaries of the trunk. Simultaneously a new layer of cells arises be- 

 tween the serous and mucous layer of the germinal membrane, at the area 

 vasculosa, and in this intercalated lamina the vascular system forms and 

 blood corpuscles appear, capillary vessels arising from the coalescence of 

 nucleated cells, the touching ends of which become pervious. As the 

 process goes forward, a network of such vessels is constructed, and it is 

 to be particularly remarked that this takes place and that the blood is in 

 circulation prior to the existence of the heart. Around the extending 

 blood-vessels or vascular area runs a circular capillary called the terminal 

 sinus in the first stage of the process, but this disappears as the vessels 

 extend all over the germinal membrane. The extension of these vessels 

 is in part accomplished by the cells from which they have arisen elon- 

 gating themselves into processes. 



Fig. 261, first appearance of blood-vessels in vascular layer of germ- 



