594 DEVELOPMENT [CHAP. XLI. 



Up to this period, the development of the human embryo is very- 

 similar to that of other vertebrate animals. 



The umbilical vesicle, about four or five lines in diameter, dimin- 

 ishes considerably in size during the third month. 



The viscid albuminous material between the chorion and amnioii 

 gradually becomes absorbed, in consequence of which these two 

 membranes approach each other, and almost come into contact; 

 but they still remain separated by a thin membrane, the tunica 

 media of Bischoff. 



The villi of the chorion increase very much in that portion of 

 its surface situated over the allantois, where the placenta is to be 

 formed. They are not absorbed from other parts, but the inter- 

 spaces between them gradually become much greater ; and as the 

 ovum increases in size, the tufts appear to be almost limited to the 

 position of the placenta, although, with care, they may be seen in 

 other parts of the surface. 



The cord gradually increases in length, and the quantity of 

 liquor amnii, in which the ovum floats, becomes greater. 



Ossification commences towards the end of the second month, 

 when the foetus is about an inch in length. The septum of the 

 heart begins to be formed, and the aortic arches diminish in 

 number to two, which unite behind to form the descending aorta. 

 One of these arches becomes the pulmonary artery. The develop- 

 ment of the kidneys, ovaries, testes, and external generative organs 

 commences. The bladder is formed somewhat later by the pinching 

 off of a portion of the urachus, a narrow tube, which is all that 

 remains of the allantois. The cavities of the mouth and nose are 

 not yet separate ; but the divisions between the fingers and toes 

 are commencing to be marked. 



During the third month, the embryo increases from one inch to 

 about two and a half or three inches in length. In the fourth, it 

 increases another inch; but during the fifth month it grows 

 so rapidly as to attain the length of twelve inches. Fat is 

 formed about this time, and the nails are developed. The first 

 movements of the foetus felt by the mother, and termed quickening, 

 usually takes place between the fourth and fifth month. The whole 

 surface is covered with lanugo or soft down. The very slow growth 

 of the foetus during the time when the organs are being evolved, 

 and it is assuming its distinctive characters, contrasts very remark- 

 ably with its rapid increase in size as soon as the various organs 

 have attained a definite form. The process of evolution is a com- 

 plicated one, and obviously requires much time for the occurrence 



