1 1 26 REPRODUCTION 



of mesoderm. So that now the body consists of a dorsal tube (the 

 neural canal), essentially of ectodermic origin, a ventral tube (the 

 alimentary canal), essentially of endodermic origin, and between the 

 two a massive double layer of mesodermic tissue, which contributes 

 supporting elements to both. At this point it may be well to emphasize 

 the fact that this embryological distinction of the three primitive layers 

 has a deep and fundamental meaning, and corresponds to a physiological 

 distinction that endures throughout life. The endoderm, the lowest 

 layer in position, may also be described as the lowest in the physiological 

 hierarchy. It furnishes the epithelial lining of the alimentary canal 

 from the beginning of the oesophagus to near the end of the rectum, as 

 well as the epithelium of the organs which arise from diverticula of the 

 primitive intestine viz., the digestive glands (with the exception of the 

 salivary glands), the lungs, and the passages leading to them, the 

 thyroid, and the greater part of the thymus gland in its primitive con- 

 dition before tha lymphoid tissue derived from the mesoderm has as 

 yet grown into it. According to some authorities, the notochord is also 

 derived from the endoderm. 



Upon the whole, it may be said that the tissues of endodermic 

 origin are essentially concerned in chemical labours, in the absorption 

 of food material and excretion of waste products. The mesodermic 

 tissues are essentially concerned in mechanical labour; they are the 

 tissues of movement and of passive support. The ectodermic tissues 

 are at the top of the pyramid; they govern the rest. 



From the mesoderm 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 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 ectoderm, 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 dis- 

 tinctly 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 textbook, such as Quain's 

 ' Anatomv.' 



Pevelopment of the Connections between the Embryo and the 

 Uterus. 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 ectodermic covering (the trophoblastic 

 envelope), consisting of two layers of cells, one unquestionably of fcetal 

 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 other* 



