THE EMBRYO. 



109 



ing of the embryo, both in an antero-posterior and a lateral direction, the original 

 ovum, with the three layers derived from the cleavage of the biastodermic mem- 

 brane which cover it. is converted into a sort of hour-glass shape with two unequal 

 globes. The smaller globe is formed by the part of the biastodermic membrane 

 (area germinativa) which has already undergone certain changes in the formation 

 of the embryo, and constitutes the part which has been compared to a canoe. The 

 larger globe is called the yolk-sac or umbilical reside, and is formed by the rest of 

 the biastodermic membrane i. e. that part which is not concerned in the formation 

 of the area germinativa. The two freely communicate through the constriction 

 Avhich is the site of the future umbilicus, and through this constriction the internal 

 layer of the biastodermic membrane (the hypoblast) and the innermost of the two 

 layers into which, as has been already stated, the mesoblast divides viz. the splanch- 

 nopleure. pass out : the incurving having only involved the somato-pleural layer of 

 the mesoblast and the epiblast (Fig. 84). The umbilical vesicle is, therefore, at 



Hypoblast 



lanchnopleure 



Splanchnopleure 

 Somatopleure 

 Pleu ro -peritoneal 

 cavity 



Neural canal 



FIG. &!. Diagrammatic section of embryo, showing the formation of 

 the umbilical vesicle. 



first a mere part of the general cavity of the yolk, partly enclosed by the embryo ; 

 but as the latter grows round on all sides toward the umbilical aperture, the 

 yolk becomes distinguished into two portions. One lies inside the embryo, and 

 eventually forms a part of the intestinal cavity (out of which also, as will here- 

 after be seen, the bladder is developed). The other lies external to the embryo 

 and remains therefore for a time a part of what is. in a more restricted sense, 

 the ovum. The two parts are almost separated from each other by the meeting 

 of the abdominal walls of the embryo at the umbilicus, through which they 

 still communicate by a passage, the omphalo-mesenteric duct, the destination of 

 which will be pointed out presently. The extra-embryonic portion is of small 

 importance and very temporary duration in the human subject. It is for the 

 purpose of supplying nutrition to the embryo during the very earliest period, 

 before it can obtain it from the uterine sinuses of the mother. In the oviparous 

 animals, however, where no supply of nutrition can be obtained from the 

 mother, since the egg is entirely separated from her, the yolk-sac is large and 

 of great importance, as it supplies nutrition to the chick during the whole of 

 fetation. Vessels developed in the middle biastodermic layer soon cover the 

 umbilical vesicle, forming the i'asciil<~ir area, the chief vessels of which are the 

 omphalo-rnesenterie. two in number (Fig. 85). The vessels of this area appear 

 to absorb the fluid of the umbilical vesicle, which as the fluid is absorbed dries up 

 and has no further function. The activity of the umbilical vesicle ceases about 

 the fifth or sixth week, at the same time that the allantois, which is the great bond 



