GENERAL FEATURES. 289 



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 which 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 gradually 

 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 rudiment of the optic 

 vesicle (op.}. 



The mesoblast on each side of the body is divided into a 

 series of segments, known as protovertebrae 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 almost the 

 extreme posterior to the anterior end of the embryo. It lies 

 between the ventral wall of the spinal canal and the dorsal wall 

 of the intestine ; and round its posterior end these two walls 

 become continuous with each other (vide fig.). Anteriorly the 



