io82 REPRODUCTION 



has not showed itself by any external token ; the embryo still appears as 

 a portion of the germi al area, and lies in its plane. But now a pocket, 

 or crease, or moat, beginning at the head as the head-fold, then pushing 

 under the tail, gradually creeps round and undermines the whole 

 embryo, which is raised above the general level, and, as it were, scooped 

 out from the rest of the blastoderm; till at length it lies on the latter, 

 something like an upturned canoe, enclosing a tube, complete in front 

 and behind, but still open in the middle, where it communicates with 

 the interior of the yolk- vesicle. Since this tube has been formed by the 

 tucking in of the three ancestral layers of the blastoderm, it follows that 

 it is lined by endoderm, supported externally by the splanchnic sheet 

 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 lavers 

 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 the lymph oid 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 

 ' Anatomy.' 



