FORMATION OF THE EMBRYO 



1127 



line of union 



prechorion 



amnon 



somatop/eure 

 coelom 



sptancfinopleure 



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 foetal 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 

 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 

 foetal 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 about 

 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 cr 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. 

 And the vitelline vessels deriving their further supply of food and 

 oxygen from the tissues of the mother in contact with the ovum cease 

 to be of use as soon as the second and more perfect placental circulation 

 is established, and soon shrivel up and disappear, as the umbilical 

 vesicle shrinks. 



The second circulation of the embryo is developed in connection with 

 a remarkable offshoot from the hind-gut called the allantois,_ which, 

 before the fifth day in the chick and during the second week in man, 

 pushes its way out between the somatic and splanchnic layers of the 

 mesoderm i.e., in the pleuro-peritoneal cavity and grows through the 



Fig. 48-'. Showing the Folds of the Somatopleure 

 in a Bird's Ovum uniting over the Embryo 

 and becoming demarcated into Amnion and 

 Prechorion (Keith). 



