102 Vertebrate Embryology 



In vertebrates the gastrula is distinctly bi- 

 laterally symmetrical, so that the future head 

 and tail ends, as well as the dorsal and ventral 

 sides, of the embryo may be recognized. 



In the chick, with which we are now espe- 

 cially concerned, we have seen that the seg- 

 mentation was confined to the small germinal 

 disc, and that it was completed by the time the 

 egg was laid. 



The first indication of segmentation that is 

 seen is a slight vertical furrow extending across 

 the centre of the germinal disc, but not reach- 

 ing quite to the sides (Fig. 34, A). Soon 

 another vertical furrow is formed at right 

 angles to the first, so that the germinal disc 

 is now divided into four equal parts (Fig. 

 34, A). Four radial furrows are next formed, 

 lying about half-way between the first two, 

 and then several cross-furrows divide the eight 

 radial segments into smaller central and larger 

 peripheral ends (Fig. 34, B). The central 

 group of smaller cells does not lie exactly in 

 the centre of the germinal disc, but a little 

 nearer to one side than to the other (Fig. 34). 



Furrows, running in all directions, now 

 appear in rapid succession, and the germinal 

 disc is soon divided into a large number of 



