CHANGES IN, AND PECULIARITIES OF, THE HUMAN FCETUS 405 



PLATE XCIV (continued) 



processes, and iris, and in Figs. A and B is still oonnected with the brain by the hollow optic nerve ; /, thickening of the corneal 

 lamina around the spot from which the lens (g) has detached itself ; h, vitreous body (after Carpenter). 



Fig. 3. — Brains of various kinds. 



A. Brain of cod. a. Olfactory lobes ; b, cerebral hemispheres ; c, middle brain giving rise to the optic nerves ; d, cerebellum ; 

 e, spinal cord. 



B. Brain of shark, lettering as in A (after Dallas). 



C. Brain of alligator, a. Olfactory ganglia ; b, cerebral hemispheres ; r, optic tubercles ; d, cerebellum ; e, medulla oblongata. 



D. Brain of rabbit, a, Olfactory ganglia ; h, cerebral hemispheres, separated to show corpora striata (c), optic thalami (d), and 

 tubercula quadrigemina (e) ; /, cerebellum ; g, spinal cord (after Dalton). The comparative anatomy of the brain illustrates the 

 repetition of parts with certain additions ; the several parts consisting of expansions of the spinal cord. 



§ 82. Changes in, and Peculiarities of, the Human Foetus. 



That the fcBtus during development assumes a great variety of forms every one conversant with the subject 

 will readily admit. The several forms, however, only indicate general plan and advance of type. They do not 

 countenance the behef that the higher plants and animals are actually manufactured from the lower ones by a 

 process of evolution extending over long periods. What the anatomist and physiologist have to deal with is the 

 completed product. It is not for them to show that the completed product has many things in common with the 

 lower plants and animals, but that it possesses properties and attributes which are only found in itself, and which 

 distinguish it from every other living thing. 



A general plan for the formation of plants and animals, an upward trend, and types are conceded. What is 

 denied, is unbroken continuity of substance, and the actual production of higher plants and animals out of lower 

 plants and animals by endless modifications in endless time. The two things are essentially different. In the 

 one case, there are differentiation and advance according to a plan and on common hues to a finish, and to a high 

 standard ; in the other, there are differentiation and advance also according to a plan and on common hues, but 

 both the differentiation and advance are partial : in other words, the higher plants and animals go all the way — the 

 lower ones only part of the way, and that the least difficult part. It matters not by what route the final goal is 

 reached ; suffice it to say, that only the highest forms can, by any possibiUty, attain it. It is the amount of differen- 

 tiation which distinguishes the higher from the lower forms, and it avails nothing to say that man during his 

 foetal existence presents featiires found in animals below him in the scale of being. The question really at issue 

 is, not what man has in common with the lower animal, but, what he has which they have not. The principle which 

 underhes mechanics is the same in all machinery, but a distinction is to be drawn between the simple and complex 

 machine. 



I have no desire to minimise or underrate the resemblances in the several grades of plants and animals, but 

 I must insist on the differences. I therefore feel it my duty to refer to the former so far as they concern man. 



During the early stages of human gestation, when the blastodermic and other membranes and parts are being 

 formed, it would be difficult to distinguish the human embryo from that of a fish, a reptile, or a bird. Then at a 

 later stage the human embryo and foetus display segmentation and a symmetrical repetition of parts met within 

 the articulata ^ and in the lower vertebrates. The segmentation in question appears in the visceral arches and in 

 the square-shaped proto-vertebrse (future spinal column). The human foetus also displays a short, rudimentary 

 tail, which is persistent in the majority of quadrupeds. Its limbs appear as buds, and some children are born 

 without limbs ; a condition which prevails in fishes and many reptiles. The arrangement of its blood-vessels and 

 circulation is, moreover, pecuhar. The distribution of the great vessels connected with the heart resembles that 

 found in the fish, amphioxus, &c. The vessels in question are arranged in five symmetrical pairs in the region 

 between the thorax and head ; certain of the symmetrical vessels ultimately disappear, and are replaced by a 

 non-symmetrical set, consisting of the aorta and pulmonary artery and their branches (Plate xcii.. Fig. 4, 

 A, B, C, D, page 396). 



The foetal circulation is further a mixed circulation, that is, the blood circulated is partly arterial and partly 

 venous ; a condition which obtains in fishes and reptiles. 



The mixed circulation is due to the imperfect formation of the fcetal heart ; the right and left auricles, which 

 contain more or less perfect venous and arterial blood, communicating with each other by means of the Eustachian 

 valve with the result that the blood forced by the auricles into the left or systemic ventricles is partly venous 

 and partly arterial. The blood is further mixed, in a minor degree, by means of the pervious ductus arteriosus. 

 The circulation in the foetus and in the placenta, and other points connected with the development of the blood-vessels, 

 heart, &c., are illustrated at Plate xcv., page 407. 



' This sub-kingdom of Cuvier has been divided into Arthropoda (Insects, Myriopoda, Malacopoda, Araehnida, Pycnogonida, Crustacea) 

 and Anarthropoda, including the Annulata and allied forms. 



