74 GENERAL FEATURES. 



G. 



During the preceding stages growth in the embryo is very- 

 slow, and considerable intervals of time elapse before any 

 perceptible changes are effected. This state of things now 

 becomes altered, and the future changes succeed each other 

 with far greater rapidity. One of the most important of these, 

 and one w^hich first presents itself during this stage, is the dis- 

 appearance of the yolk spherules from the embryonic cells, and 

 the consequently increased transparency of the embryo. As a 

 result of this, a number of organs, which in the earlier stages 

 were only to be investigated by means of sections, now become 

 visible in the living embryo. 



The tail-swellings (t. s.) are still conspicuous objects at the 

 posterior extremity of the embryo. The folding off of the 

 embryo from the yolk has progressed to such an extent that it 

 is now quite possible to place the embryo on its side and 

 examine it from that point of view. 



The embryo may be said to be attached to the yolk by 

 a distinct stalk or cord, which in the succeeding stages gra- 

 dually narrows and elongates, and is known as the umbilical 

 cord (so.s.). The medullary canal has now become completely 

 closed, even in the region of the brain, where during the last 

 stage no trace of a medullary groove had appeared. Slight 

 constrictions, not perceptible in views of the embryo as a 

 transparent object, mark off three vesicles in the brain. These 

 vesicles are known as the fore, mid, and hind brain. From the 

 fore-brain there is an outgrowth on each side, the first rudi- 

 ment of the optic vesicle (oj).). 



The mesoblast on each side of the body is divided into 

 a series of segments, known as protovertebrse or muscle-plates, 

 the first of which lies a little behind the head. The mesoblast 

 of the tail has not as yet undergone this segmentation. There 

 are present in all seventeen segments. These first appeared 

 at a much earlier date, but were not visible owing to the 

 opacity of the embryo. 



Another structure which became developed in even a 

 younger embryo than C is now for the first time visible in 

 the living embryo. This is the notochord : it extends from 



