FORMATION OF THE EMBRYO 



1083 



Development of the Connections between the Embryo and the 

 Uterus. In the first period of its development the ovum, nestling in 

 the pouch formed by the decidua serotina and reflexa, is fed from the 

 maternal blood and tissues directly, without the mediation of foetal 

 bloodvessels, through the finger-like processes or villi with which its 

 external layer, the zona pellucida, becomes studded. At the earliest 

 stage at which a human ovum has been studied after implantation it is 

 already enveloped by a thick ectodermic covering (the trophoblastic 

 envelope), consisting of two layers of cells, one unquestionably of foetal 

 origin, the so-called cells of Langhans, and the other the syncytium, the 

 origin of which is assigned by some authorities to the ovum, by others 

 to the maternal tissues. The trophoblastic covering is everywhere in 

 contact with the maternal blood, which, pushing its way into the tropho- 

 blast at intervals, divides it into columns. Later on the fcetal mesoderm 

 grows into these, and so the primary chorionic villi are formed. It is 

 not till after the first three weeks that bloodvessels make their way into 

 these villi, although the mesoderm of the foetus begins to enter the villi 

 about the end of the first, or the beginning of the second, week. The 

 scanty yolk of the human 



line of union 



prechorion 



etobryo 

 H \ amnion 



somatopfeure 

 coelom 



spfanchnopleure 



ovum is totally inade- 

 quate to supply it with 

 nutriment for the time 

 that elapses before the 

 bloodvessels are devel- 

 oped, and food sub- 

 stances must be obtained 

 from the maternal liquids 

 by imbibition, osmosis, 

 diffusion, or filtration, 

 aided, perhaps, by more 

 special absorptive pro- 

 cesses on the part of the 

 fcetal tissues. Soon the 

 heart appears as a tube 

 (at first double), formed 

 by cells belonging to the 

 splanchnic layer of the 

 mesoderm. It begins to 

 pulsate in the chick as 

 early as the middle of the 

 second day, although it as 



yet contains neither nerve-cells nor fully-formed muscular fibres. In 

 the mammal pulsation is late in making its appearance, in man abou t 

 the beginning of the third week. A bloodvessel grows out from the 

 anterior end of the heart and divides into two primitive aortic arches, 

 from each of which a vessel (omphalo-mesenteric or vitelline artery] runs 

 out in the mesoderm covering the umbilical vesicle, or yolk-sac. The 

 blood is returned to the heart by the vitelline veins coursing in on the 

 walls of the vitelline duct. In this way the store of nutriment in the 

 umbilical vesicle of the chick, which is the only solid or liquid food it 

 receives or needs during the whole period of development, is tapped, 

 and a regular channel of supply established. Oxygen is at the same 

 time absorbed through the porous shell; but later on this respiratory 

 function is taken over by the second or allantoic circulation. In the 

 mammal the circulation on the umbilical vesicle is of much less conse- 

 quence, for the quantity of material left over after the formation of the 

 blastoderm is exceedingly small ; it is only with a few days' provision 

 in its haversack that the embryo starts out on its developmental march. 



Fig. 461. Showing the Folds of the Somatopleure 

 in a Bird's Ovum uniting over the Embryo 

 and becoming demarcated into Amnion and 

 Prechorion (Keith). 



