4 



68 OBSTETRICAL PHYSIOLOGY. 



wards the median line ; the brain and spinal marrow, which are its ear- 

 liest 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 condi- 

 tion 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 : (i) 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 anfiexes, or envelop- 

 ing membranes. The different 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 epi- 

 dermis and the organs of sense ; the middle layer has l3een named the 

 vascular or germifiative, as it contains the principal vessels of the em- 

 bryo, and the locomotory organs are developed in its substance ; while 

 the internal layer is known as the mucous or intestino-glandular, from its 

 constituting the mucous membranes, its principal portions forming the 

 intestines and glands. Each of these layers furnishes, in the course of 



J 



Fig. 33. 

 Ovum Twekty to Twenty-five Days Old. 



I, Villosities of the yitelline Membrane; 2, External Layer of the Blastoderm, or Second 

 Chorion, with its Villosities ; 3, Umbilical Vesicle, formed by the Inner Layer of the Blasto- 

 derm ; 4, Vessels of the Umbilical Vesicle ; 5, Cephalic and Caudal Processes ; 6, Embryo ; 

 7, Allantoid Vesicle. 



its development, the intra-foetal and the extra-foetal parts. We shall ex- 

 amine the latter first ; merely, noting in the mean time, that the intra- 

 foetal 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 — such as 

 the retina and the labyrinth of the ear; while the extra-foetal 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 



