lo io A MANUAL OF PHYSIOLOGY 



tissues ; and the Wolffian body and its appendages, which are the 

 predecessors of the genital glands and ducts, and of the chief portion 

 of the renal apparatus. 



The ectoderm forms the epidermis and its appendages, the epithelial 

 end-organs of the nerves of special sense, and the nervous system, 

 cerebro-spinal and sympathetic. The salivary glands and the 

 mucous lining of the mouth and anus are developed from the ecto- 

 derm, which is indented to meet the intestinal canal and give it 

 access to the exterior at either end. 



It is not possible here to trace in detail the development of all the 

 organs of the embryo. Its nutrition and metabolism not only 

 distinctly belong to the physiological domain, but, carried on as 

 they are under conditions that seem so strange, and even so bizarre, 

 to one acquainted only with adult physiology, are calculated to 

 throw light on the metabolic processes of the fully-developed body. 

 And they cannot be understood without reference to the peculiarities 

 of the vascular system in foetal life. These w r e shall accordingly 

 describe, but for further details as to the anatomy of the embryo the 

 student is referred to some standard anatomical text-book, such as 

 Quain's ' Anatomy.' 



Physiology of the Embryo. In the first period of its development 

 the ovum, nestling in the pouch formed by the decidua serotina and 

 reflexa, is fed from the maternal blood and tissues directly, without 

 the mediation of foetal bloodvessels, through the finger-like processes 

 or villi with which its external layer, the zona pellucida, becomes 

 studded. At the earliest stage at which a human ovum has been 

 studied after implantation it is already enveloped by a thick ecto- 

 dermic covering (the trophoblastic envelope), consisting of two layers 

 of cells, one unquestionably of foetal origin, the so-called cells of 

 Langhans, and the other the syncytium, the origin of which is assigned 

 by some authorities to the ovum, by others to the maternal tissues. 

 The trophoblastic covering is everywhere in contact with the maternal 

 blood, which, pushing its way into the trophoblast at intervals, 

 divides it into columns. Later on the foetal mesoderm grows into 

 these, and so the primary villi are formed. It is not till after the 

 first three weeks that bloodvessels make their way into these villi, 

 although the mesoderm of the foetus begins to enter the villi about 

 the end of the first, or the beginning of the second, week. The 

 scanty yolk of the human ovum is totally inadequate to supply it 

 with nutriment for the time that elapses before the bloodvessels are 

 developed, and food substances must be obtained from the maternal 

 liquids by imbibition, osmosis, diffusion, or filtration, aided, perhaps, 

 by more special absorptive processes on the part of the foetal tissues. 

 Soon the heart appears as a tube (at first double), formed by cells 

 belonging to the splanchnic layer of the mesoderm. It begins to 

 pulsate in the chick as early as the middle of the second day, although 

 it as yet contains neither nerve-cells nor fully-formed muscular fibres. 

 In the mammal pulsation is late in making its appearance, in man 

 about the beginning of the third week. A bloodvessel grows out 

 from the anterior end of the heart and divides into two primitive 

 aortic arches, from each of which a vessel (omphalo-mesenteric or 

 vitelline artery] runs out in the mesoderm covering the umbilical 

 vesicle, or yolk-sac. The blood is returned to the heart by the 

 vitelline veins coursing in. on the walls of the vitelline duct. In this 

 way the store of nutriment in the umbilical vesicle of the chick, 

 which is the only solid or liquid food it receives or needs during the 



