GENERAL EMBRYOLOGY 1639 



septa between the blood-lacunae ; and they receive their blood from 

 these sinuses through minute pores, by which the basal layer and 

 septa are perforated. Some of these veins pass to the uterine 

 surface or vault of the placenta, and thence to the veins of the 

 uterine wall. Most of them, however, pass to the periphery of the 

 placenta, where they open into a channel, called the circular or 

 marginal venous siniis, from which it passes into the veins of the 

 decidua vera. 



Tlie Placenta and Deciduous Membranes at Full Term. — ^At the 

 period of birth (full term) rupture of the amnion takes place as a 

 rule, and this is accompanied by the discharge of the liquor amnii. 

 Expulsion of the child then takes place, but the placenta and the 

 deciduous membranes usually remain within the uterus for a certain 

 period. 



The umbilical cord having been divided, the next stage consists in 

 the expulsion of the placenta and membranes. These membranes 

 are as follows: (i) The amnion; (2) the chorion leve; and (3) the 

 united decidua capsularis or reflexa, and stratum compactum of 

 the decidua vera. The parts of the uterine mucosa which remain 

 in situ or in utero are (i) the stratimi spongiosimi, in whole or in 

 part, and (2) the stratum profundum of the decidua vera and of the 

 decidua basalis or serotina, with the parts of the glands contained 

 within these two strata. 



The uterine mucosa therefore undergoes a spiiiiing, usually at 

 the line of junction of the stratum compactum and stratum 

 spongiosum, the former being expelled, whilst the latter remains 

 in situ. The splitting, however, may take place through the stratum 

 spongiosum. 



The impoverished uterine mucosa now undergoes regeneration, 

 the cast-off stratum compactum being formed anew by the epithelial 

 cells which line the gland-cavities within the stratum spongiosum 

 and the deep ends of the uterine glands within the stratum pro- 

 fundimi. 



The body of a child at birth is usually smccired with a white 

 cheesy coating, which is called the vernix caseosa {smegma embryo- 

 num). This is formed by the sebaceous glands of the skin, and it 

 contains epidermic scales, which have become detached from the 

 superficial layer of the epidermis, known as the epitrichium. It acts 

 as a kind of ointment, which protects the epidermis from the 

 softening action of the hquor amnii, and it occurs in greatest 

 abundance in the following regions : the head and face, the flexures 

 of the joints, the palms of the hands, the soles of the feet, and the 

 natal cleft. 



The fine soft do\\Ti or hair of the child at birth, some of which is 

 shed prior to birth, imparts a somewhat woolly appearance to the 

 regions provided with it. This foetal down or hair is called the 

 lanugo. 



Relative Position of Structures within the Gravid Uterus. — ^When 

 the wall of the body of the gravid uterus is divided in the operation 



