I'EVKlJiVMEST <>F rilE KM /!/:)'<>. 



73 



extra-fcEtal parts. We shall examine the latter first ; merely noting, 

 in the meantime, that the intra-fcetal parts of the external or sensitive 

 layer of the blastoderm form the epidermis and its appendages — as the 

 hair, claws or hoofs, glands of the skin etc. — the central nervous 

 system, and the organs of sense — sucli as the retina and tlie labyrinth 

 of the ear ; while tlie extra-foctal parts it forms are due to its alteration 

 in shape. 



The foetus, as we have seen, is a circular body applied against a 

 certain point of the blastoderm, whose outer layer is continuous, and 

 extends over the internal face of the chorion and the surface of the 

 embryo without any limit. But the inflection or bending of the latter 

 at its two extremities causes the external layer to become depressed, 

 and to constitute two folds ; one investing the head, the other the 

 opposite extremity of the body. The lateral parts of the embryo are 

 also inflected towards each other to form the thoracic-abdominal cavity. 





r^. 



tr-vj^ •. 



Fig. 43. 



OvcM Twenty to Twenty kive Days Old. 



1, VUlositie.s of the Vitelline Membrane ; 2, External Layer of the Bla.stoilerni, 

 or .Second Chorion, with its Villosities ; 3, Uiiiliilical Vesicle, formed by 

 the Inner Layer <>f the Blastoderm ; 4, Vessels of the Umbilical Vesicle ; 

 5, Cephalic and Caudal Processes ; 6, Embryo ; 7, Allantoid Vesicle. 



and it is thus enclosed in the serous layer, whose sides soon meet above 

 the back of the young creature, and a short pedicle — the superior 

 umbilicus — joins the two portions of the blastoderm. This pedicle soon 

 disappears, and the embryo is then enclosed in an independent sac — the 

 amnion already referred to. The sensitive layer is, tlicrcfore, decom- 

 posed into two sections : an internal, the amnion ; and an external, the 

 serous vesicle, which is applied against the inner face of the zona 

 pellucida, and concurs to form the cliorion. 



The middle layer will be noticed hereafter, and we now go to the 

 internal layer. The intra-fcetal parts of this so-called " mucous layer " 

 form the epithelium of the intestines and the glands belonging thereto, 

 the respiratory apparatus, the kidneys, and the bladder. These organs 

 are developed at the same time as the extra-fcrtal parts. 

 ♦^ In curving in upon itself, the embryo encloses a portion of the internal 

 blastodermal layer, but the union between the free and the imprisoned 

 parts is at first largely maintained ; soon, however, it contracts, and in 



