10 COMPARATIVE ANATOMY. 



tube; the latter surpasses the former considerably in size, and, 

 as it encloses the viscera, it may be called the visceral tube. 



A cellular, cartilage-like rod the notochord (chorda dor- 

 sal is), arising primitively as an axial thickening of the hypoblast 

 (Fig. 7, A and B, Ch, Ch l ), forms the basis of the vertebral column, 

 that is, the segmented axial skeleton which characterises the 

 Vertebrate body. 1 This segmentation of the axis, as well as of 

 other organs and systems of organs (musculature, ribs, roots of 

 spinal nerves, sympathetic cord, pro- and meson ephros), indicates 

 that the Vertebrata must have arisen from an invertebrate and 

 segmented ancestral form. 



The anterior ends of the enlarged medullary cord and alimentary 

 tract enter into a close relation with the outer world, the former 

 giving rise to the brain and to certain parts of those sense-organs 

 with which the higher cerebral functions are connected, while from 

 the latter are developed the mechanisms for the taking in of 

 nutriment and for respiration. 



The anterior section of the embryo, or head, passes behind 

 into the trunk, in the hinder part of which the anal and urinogenital 

 apertures are situated. These parts are classed together as the 

 body-axis, as distinguished from the limbs, or appendicular 

 organs, which arise from the trunk. 



In Reptiles, Birds, and Mammals, a delicate investment, the 

 arnnion, is early formed round the embryo; it arises as a fold of 

 the somatopleure (Fig. 9, AF, A). A sac-like out-growth from 

 the hinder part of the primitive intestine (i.e. from the splanchno- 

 pleure) gives rise to the allantois (At) which becomes highly 

 vascular, and in Reptiles and Birds extends round the embryo 

 close under the egg-shell; it here serves as an embryonic respiratory 

 organ. In all Mammals, except Monotremes and Marsupials, the 

 allantois becomes attached to a definite region of the uterine wall, 

 and from it vascular processes or villi grow out into crypts of the 

 latter, which is also plentifully supplied with blood-vessels. Thus 

 a placenta is formed, in which interchanges can take place both 

 as regards nutritive materials and aeration between the blood of 

 the mother and that of the foetus. 



Considerable differences are observable in the form of the placenta in dif- 

 ferent Mammals. The most primitive arrangement is most probably one in 

 which the allantois becomes attached along a discoidal region of the wall of 

 the uterus, and the various modifications seen in the different groups may be 

 looked upon as having arisen in order to increase the absorptive surface. This 

 may be effected either by the area of that part of the allantois which is covered 

 by placental villi becoming extended, or by the increase in complexity of the 

 villi and crypts. In the latter case, the interlocking between foetal and maternal 

 parts is so close that the mucous membrane of the uterus is torn away with 

 the foetal part of the placenta at birth, and the latter is then said to be 

 deciduate. In the former case, the discoidal placenta may -extend so as to 



1 In the lowest Vertebrates, the segmentation of the body is indicated mainly by 

 somites. 



