42 OBSTETRICS. 



The parts first formed are those which most clearly distinguish 

 the vertebrated animals from all others, viz., the vertebral column 

 and spinal cord. These first make their appearance in the situation 

 of the embryonic cell, and are included under the name of chorda 

 dorsalis, which is found to be composed, wherever it exists, of nu- 

 cleated cells. From cells exterior to this, is developed the vertebral 

 column. Concurrently with this development, appears the vascular 

 system, which is first seen in the middle layer of the germinal mem- 

 brane, called the "vascular. Vessels are formed here which serve to 

 take up the nutriment supplied by the yelk, and carry it to the tis- 

 sues of the embryo. These vessels are first seen in that part of the 

 vascular lamina of the germinal membrane which immediately sur- 

 rounds the embryo, and they form a delicate network of vessels 

 called the vascular area ; this gradually extends itself till the vessels 

 spread over the whole of the germinal membrane, and it is through 

 their agency that the nutritious matter of the yelk is conveyed to the 

 embryo. The vessels of the yelk-bag terminate in two large trunks 

 called omphalo-mesenteric, meseraic, or vitelline vessels, which enter 

 the embryo at the point which afterwards is known as the umbilicus. 

 The first movement of fluid takes place towards the embryo, and 

 may be discovered before any heart is seen. The heart is formed in 

 the substance of the vascular layer, by a dilatation of the trunk, into 

 which the blood-vessels unite. It is at first a mere excavation, but 

 afterwards its walls become more developed, and it is divided into 

 cavities. Along with the development of the vascular, appears also 

 the permanent digestive cavity ; this originates in the separation of a 

 small portion of the yelk-bag lying immediately beneath the embryo 

 by a doubling in of the mucous layer, so as to enclose a cavity. 

 This, by subsequent prolongation and involution of its walls, is ren- 

 dered more complex, so as to form a stomach and intestinal tube. 

 The digestive cavity communicates for some time with the yelk-bag 

 (from which it has thus been pinched off) by the opening that is left 

 by the imperfect meeting of the folds of the germinal membrane 

 that forms its walls. In the mammalia, this orifice is gradually nar- 

 rowed, and at last completely closed ; and the yelk-bag thus sepa- 

 rated is afterwards thrown oft'; it is then known as the umbilical 

 vesicle^ and may be seen upon the umbilical cord up to a late period 

 of pregnancy. 



At first the body of the embryo is more elongated than afterwards, 

 and the trunk is the first part developed, at the upper extremity of 

 which is a small prominence less thick than the rest, and separated 

 by an indentation, which distinguishes the head. No traces of ex- 

 tremities can yet be discovered, or of any other prominent parts. 

 It is straight, or nearly so, the posterior part being slightly convex, 

 the anterior concave. 



Afterwards the head becomes more rapidly developed, so that at 



