642 



REPRODUCTION. 



layer, resulting from the consolidation of two others, then constitutes 

 the external investing membrane of the egg. 



The allantois, in the mean time, increases in 

 size and vascularity. Still following the course 

 of the amniotic folds, it insinuates itself between 

 them, until it comes in contact with the external 

 membrane above described. It then begins to 

 expand laterally, growing round the body of the 

 embryo, and bringing its vessels into contact with 

 the external investing membrane of the egg. 



By a continuation of this process the allantois 

 completely envelops the body of the embryo ; its 

 J" opposite borders coming in contact and fusing 

 tois nearly complete. a. with each other over the dorsal region, in the 

 inner lamma of ammotic game manner as the amniotic folds had previouslv 



fold. o. Outer lamina ot J 



amniotic fold. c. Point done (Fig. 197). It lines the whole internal sur- 



t fa e ^niotic folds face of the investing membrane with a flattened, 



lautois is seen penetrating vascular sac ] its blood-vessels coming from the 



between the inner and interior of the body of the embryo, and its cavity 



outer laminae of the am- . * * * 



niotic folds. still communicating with that of the intestinal 



canal. 



It is evident, accordingly, that there is a close connection between 

 the formation of the amnion and that of the allantois. It is by this 

 means that the allantois, which is originally 

 an extension of the internal blastodermic 

 layer, comes to be situated outside the em- 

 bryo and the amnion, and is brought into 

 relation with surrounding media. The two 

 laminae of the amniotic folds, by separating 

 from each other as above described, open a 

 passage for the allantois, through which it 

 comes in contact with the external membranous 

 investment of the egg. 



Physiological Action of the Allantois. 

 EGG, with the allantois fully The physiological action of the allantois, in 



formed. a. Umbilical vesicle. . . J , 



b. Amuion. c. Allantois. " simplest character, may be studied with 

 advantage in the fowl's egg, where it forms 



an extensive and highly vascular organ, without important modifica- 

 tion of its original structure. 



The egg of the fowl contains, when first laid, an abundant deposit 

 of semi-solid albuminous matter in which the yolk is enveloped. This 

 affords, in connection with the yolk, a sufficient quantity of moisture 

 and organic nutriment for the growth of the embryo. The necessary 

 warmth is supplied by the fowl in incubation ; and the atmospheric 

 gases can pass without difficulty through the porous shell and its 

 lining membranes. On the commencement of incubation, a liquefac- 

 tion takes place in the albumen above the blastoderm ; allowing the 



FIG. 197. 



Diagram of the FECUNDATED 



