236 Professor Clark on a Case 



ally increasing ovum, it becomes thinner — is brought in contact 

 with its external portion (the original decidua) — and finally, about 

 the fourth month of pregnancy, coalesces with it. 



At an early period after the egg has passed into the uterus, 

 the granular cortical membrane expands — its granules assume an 

 arboresent filamentous form, and thus, penetrating the reticula- 

 tions of the reflected, fix it to the proper decidua and so to the 

 uterus. Since the ovum increases considerably in size, previous to 

 this attachment, the earliest nutrition of the embryo, is to be at- 

 tributed to absorption from the fluids of the tube and of the 

 uterus, by the granular exochorion. The series of changes by 

 which it is perfected takes place in the blastoderma. These have 

 not been very accurately ascertained, in the earliest stage, in the 

 ova of mammalia : but there is ever reason to believe that they 

 are exactly analogous to those which have been determined with 

 much care in the hen's egg. 



The first change is the gradual separation of the blastoderma 

 in the region of the disc into what now appear to be its compo- 

 nent parts, whilst at the same time it recedes a little from the 

 yolk. It presents an external serous layer, and an internal mu- 

 cous layer. At the same time the proportional distribution of 

 these membranes is different in different parts of the disc: thus 

 in the center the serous portion prevails more than the mucous, 

 and in the circumference the mucous portion more than the serous. 

 Hence the disc presents a pellucid area, surrounded by a broad 

 darker ring. Between these two laminae, a third portion of the 

 blastoderma gradually becomes evident. It appears as a layer of 

 granular matter: and since this matter is the substance from 

 which the blood vessels and their contents are formed, it is called 

 by Pander the vascular layer. There are thus three parts of the 



