72 OBSTETRICAL PHYSIOLOGY. 



thickens at the front part, to form the first appearance of the skull, and 

 the fluid between the dorsal laminae is in larger quantity, in corre- 

 spondence with it ; so that the central parts of the nervous system and 

 their coverings are laid down at the same time and grow simultane- 

 ously. The separation between the spinal cord and brain takes place 

 early, being coincident with a curving downwax'ds, towards the yolk, of 

 the anterior part of the laminae dorsalis, which defines the limit between 

 the skull and spine, brain and cord. 



Next follows the closing of the dorsal laminae over the fluid which is 

 to constitute the brain and cord. Two other laminge are in the mean- 

 time proceeding from the axis of the embryo, one on each side. They 

 grow out laterally, and tend to converge in the median line, as did the 

 dorsal laminae ; but they form a larger curve, and follow a diiferent 

 direction, converging to meet heloic the axis, where they join, except 

 at the umbilicus. 



After the rudiments of organic life have been commenced in the 

 central portion of the serous layer, a fold of its peripheral portion 

 arches over the dorsal surface of the embryo, so as to represent a sac 

 whose opening is at the edge of the fold. The opening gradually 

 decreases until the opposing folds of membrane are in contact ; it then 

 disappears, leaving the fcetus surrounded by two membranes. The one 

 next the foetus is the amnion, and the other is gradually separated from 

 the amnion, and joins the serous lamina of the blastoderm, forming the 

 " false amnion " of Pander, or the " serous covering" of Von Baer. The 

 membrane surrounding the vitellus or yolk is very vascular ; it becomes 

 oval in shape, and more pointed when it is in contact with the embryo, 

 until at length it contracts into a narrow duct, constituting the vesicula 

 alba and duct. Thus, then, we have seen the embryo developed in 

 the layers of the blastoderm, and formed by a gradual closing in of the 

 laminae towards the median line ; the brain and spinal marrow, which 

 are its earliest rudiments, are covered in, and the parts anterior to the 

 spine — the thorax, abdomen, etc. — are formed. 



We will now direct our attention to changes occurring elsewhere, and 

 return to the development of the embryo again. 



Towards the twelfth day, in the higher orders of animals, the chief 

 modifications which have just been described as occurring in the con- 

 dition of the ovum after fecundation, are accomplished. The ovum 

 then measures from one-third to one-fourth of an inch in diameter, and 

 is composed in reality of four layers or shells, enclosed one within the 

 other, but only three of which are complete; they are: (1) the yolk 

 membrane ; (2) the external layer of the blastoderm ; (3) the middle 

 layer ; (4) the inner layer. 



By ulterior modifications, the layers of the blastoderm form 

 the various organs of the foetus, and what have been termed its 

 annexes, or enveloping membranes. The diiferent layers have also 

 received other names than those mentioned, according to their functions: 

 thus, the external or serous has been named the sensitive layer, because 

 it originates the epidermis and the organs of sense ; the middle layer 

 has been named the vascular or germinative, as it contains the principal 

 vessels of the embryo, and the locomotory organs are developed in its 

 substance ; while the internal layer is known as the mucous or intestino- 

 glanclular, from its constituting the mucous membranes, its principal 

 portion forming the intestines and glands. Each of these layers 

 furnishes, in the course of its development, the intra-foetal and the 



