REPRODUCTION 869 



From the mesoblast arise the muscles, the entire vascular system 

 with its blood and lymph corpuscles, the bones and connective 

 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 epiblast 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 epiblast, 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 we 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 develop- 

 ment the ovum, nestling in the pouch formed by the decidua 

 serotina and reflexa, is fed simply by imbibition through the hollow 

 finger-like processes or villi with which its external layer, the zona 

 pellucida, becomes studded. Soon the heart appears as a tube (at 

 first double), formed by cells belonging to the splanchnic layer of 

 the mesoblast. 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 

 mesoblast 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 whole period of development, is tapped, and a 

 regular channel of supply established. Oxygen is at the same time 

 absorbed through the porous shell ; but later on this respiratory 

 function is taken over by the second or allantoic circulation. In the 

 mammal the circulation on the umbilical vesicle is of much less 

 consequence, for the quantity of material left over after the lormation 

 of the blastoderm is exceedingly small ; it is only with a few days' 

 provision in its haversack that the embryo starts out on its develop- 

 mental march. And the vitelline vessels deriving their further 

 supply of food and oxygen from the tissues of the mother in contact 

 with the ovum, cease to be of use as soon as the second and more 



