1030 EMBRYOLOGY. 



This division is summed up in the following table : — 



I_,._ ( Mare. 



Diffuse < g 

 Local and circular . | ^' ^ ' 

 I Oat. 



,,,,., , ^ ( Convex cotyledons . oi. ' 

 Multiple placenta | Concave cotyledons . f'^ 



[ (jrOitt. 



Comparison of the Annexes of the Human Fcetds with those of Animals. 



The human foetus, like that of the domesticated Mammals, is enveloped by an amnion and 

 chorion, which are generally identical in disposition with those already described. The umbilical 

 vesicle submits to the same change as in the Mare, becoming so quickly atrophied that scarcely 

 any traces of it can be found at birth. It is impossible to isolate the aUantois from the inner 

 face of the amnion and chorion ; so that some anatomists only admit its presence by analogy 

 witli what is observed in animals. 



The umbilical cord offers nothing particular. The placenta is circular ; its diameter at the 

 termination of pregnancy being from 6 to 8 inches, and its thickness from 1 to H inch. There 

 is distinguished the foetal placenta, to the midst of which the umbilical cord reaches; and the 

 maternal placenta, the villi of which dovetail with those of the former. 



The insertion of the placenta takes place towards the fundus of the uterus, near one of the 

 Fallopian tubes. When there are more than one foetus, there is a corresponding number of 

 placentas. 



Independently of those annexes — which are the same in all — there is described for the 

 human foetus a special envelope, external to the chorion — this is the membrana decidua. This 

 is formed by the hypertrophied mucous membrane, which is doubled around tlie ovum, when 

 the latter is lodged in the uterine cavity. Consequently, there results the division of the 

 decidua into two parts— the true decidua (decidua vera) which covers the uterus, and the 

 reflected decidua (decidua reflexa) which envelops the chorion. These present the characters 

 of the hypertropliied uterine mucous membrane. 



CHAPTER III. 



DEVELOPMENT OF THE FCETUS. 



In the two preceding chapters, we have seen how the fecundated ovum is modified 

 to furnish the earliest lineaments of the foetus and the organs annexed to it ; 

 this chapter will be deveted to an examination of the manner in which the foetus 

 is developed, though this subject more properly belongs to physiology. 



The young creature is designated an embryo during the early period of gesta- 

 tion, before it has assumed any definite shape ; but as soon as it exhibits the 

 form of the species to which it belongs — and particularly when the placental 

 circulation is established — it is named /a^ws. 



Grurlt has divided the duration of gestation into seven periods, the length 

 of which varies according to the species. He has also measured the dimensions 

 of the ovum, the embryo, and the foetus of each of them. We may, therefore, 

 accept the figures he has given in order to learn the age of the foetus. They are 

 shown in the two following tables : — 



