390 DESIGN IN NATURE 



PLATE XO 

 Plate xc. illustrates development in the bird, fish, and mammal. 



Fig i._A. Egg of fowl at early stage of incubation. Sliows unformed body of chick (a), with area vasculosa (h) extending 

 on either side and partly covering the vitellus (c) ; all being bounded by the twisted chalaziferous membrane {d,d'). ^ , , 



B The same further advanced. Head and body of chick (a) becoming defined ; the body being developed by amnion at 6, and 

 bavins the umbilical vesicle (d) depending from its under surface. The vascular allantois curves out from the under surface of the 

 body, and spreads above it like an umbrella (c). The allantois acts as a lung to the chick, and the umbilical vesicle as a, stomach 

 (after Dalton). 



Flu. 2.— Egg of fish, showing the circulation in the vitellus or yolk (Dalton). 



Pig. .3. — Young fish, showing formation of umbilical vesicle. 



A. Vesicle seen depending as an open sac from ventral surface. Contains the food of the young fash. 



B. Umbilical vesicle more closed and communicating with future alimentary canal. 



C. Umbilical vesicle of human embryo at fifth week (after Dalton). 



Fig. 4.— a. Impregnated egg of fowl with allantois completed, a, Umbilical vesicle ; h, amnion ; c, allantois. 

 B. The same further advanced ; alimentary canal beginning to make its appearance. <i, Umbilical vesicle ; h, allantois (after 

 Dalton). 



FlQ. 5.— The same still further advanced; the alimentary canal appearing as a convoluted tube, a, Pharynx, and proceeding 

 from it the oesophagus, stomach, alimentary canal, and bladder ; h, heart ; i; mesenteric arteries ; d, umbilical vesicle and blood- 

 vessels thereof, greatly reduced ; e, large umbilical arteries passing to placenta (/) (after Dalton). 



Fig. 6.— Section of human uterus with the impregnated ovum {b) lodged in the mucous membrane {decidua vmi) (after Dalton). 



Fig. 7.— Human uterus laid open to show the vagina, os uteri, interior of uterus, right Fallopian tube, &c. The darts indicate 

 the direction of travel of the male and female elements (spermatozoid and ovum) after coitus. <t, Fundus of uterus ; h, right ovary ; 

 r, fimbriated extremity of the right Fallopian tube which seizes the ovum as it escapes from the ovary ; (/, u, right Fallopian tube 

 laid open— shows route taken by ovum on its way to the uterus (see darts) ; /, os uterus laid open— the dart shows the route taken 

 by the spermatozoon ; the route is further indicated by the dart ; y, the male and female elements meet either in the Fallopian 

 tube, or in the uterus ; h, poi-tion of vagina ; i, os uteri. 



FiG. 8.— A. Human ovum, about the end of the first month. .;, Umbilical vesicle ; Ji, amnion ; i; shaggy chorion ; d,, amniotic 

 space ; e, body of embryo. 



B. Human ovum at the end of the third month. «, Remains of shaggy chorion with villi developed to form the foetal portion of 

 the placenta ; h, chorion devoid of villi (after Dalton). 



§ 78. Subjects connected with the Development of the Human Embryo and Foetus. 



The formation of the human embryo and foetus is, in some respects, more remarkable than that of the chick, 

 inasmuch as the relation between the parent and oflspring is much more intimate and of longer duration, and 

 involves a double set of changes, namely, changes in the mother and changes in the progeny. The relations referred 

 to, moreover, do not suddenly cease at parturition as in the case of the chick. 



The impregnated human ovum, as happens in the bird, carries with it its own pabulum until it reaches and is, 

 so to speak, grafted on the mucous lining of the uterus. Arrived there it is nourished by a process of osmosis by 

 uterine fluids and gases, namely liquor sanguinis, oxygen, and glandular secretions. The nourishing process is largely 

 mechanical, but in no sense due to irritability or artificial stimulation. Soon a series of the most delicate, involved, 

 and far-reaching changes are inaugurated. The tender embryo is to be protected and housed as well as fed, and the 

 most elaborate preparations are made for both purposes. The os uteri is plugged with mucus to guard against 

 danger from without. Two thin but tough membranes or envelopes (the amnion and chorion) containing fluids are 

 provided, so that the embryo may float daintily and freely, and be protected from any injury which may overtake 

 the parent. 



The ovum increases its absorbing surfaces by developing villi or hair-like processes on the chorion or outside 

 membrane, which may very aptly be compared to the roots of a tree (Plate xcii., Fig. 1, F, G, H, I, J, L, page 396). By 

 these processes it, at a later period, estabhshes a connection with the circulation of the mother by which it is nourished, 

 and by which its own blood, when formed, is aerated (Plate xcv., Figs. 1, 2, 3, and 4, page 407). 



The period of gestation in the human female extends over nine calendar months, and during that protracted 

 term a series of the most astounding developments which it is possible to imagine occur. The impregnated ovum, 

 from being a tiny round speck about the size of a small pin head, consisting of a vitelline membrane, vitellus, 

 germinative vesicle, and germinative spot, slowly, but surely, assumes the human form. 



In the human female child, long before the age of puberty (indeed at birth), a pair of ovaries crowded with 

 immature ova make their appearance. At puberty certain of the ova or eggs, fully matured, burst in succession 



