GENERAL EMBRYOLOGY 1637 



somatic mesoderm. This core contains an arteriole, capillaries, and 

 a venous radicle. As the arteriole traverses the mesodermic core 

 of the \'illus it gives off minute twigs, which form a rich capillary 

 network, lying close beneath the surface, from which the blood 

 passes into the venous radicle. At the free extremity' of the villus 

 the arteriole is connected with the commencement of the venous 

 radicle by a capillary loop. 



Maternal Part of the Placenta. — ^The maternal or uterine part 

 forms a small portion of the entire placenta, corresponding to the 

 ■convex uterine surface or vault, and it is developed from the decidua 

 basalis or serotina of the uterine mucosa. The decidua basalis or 

 serotina consists, at an early period, of the three strata which 

 characterize the decidua vera — namely, the stratum compactum, 

 stratum spongiosum, and stratum profundum. As the ovum sinks 

 into the decidua basalis, in order to grip the uterine mucosa, the 

 <iecidua basalis undergoes certain important changes in gi\*ing rise 

 to the maternal part of the placenta. Its ciliated columnar epi- 

 theliiim disappears, as well as the superficial part of the stratum 

 compactum and the portions of the uterine glands contained therein. 

 There are thus left (t) the deep part of the stratum compactum, 

 ^2) the stratum spongiosum, and (3) the stratum profundum. 



The deep pari of the stratum compactum gives rise to the maternal 

 part of the placenta. The portions of the uterine glands within it 

 disappear, and the bloodvessels become distended, a condition of 

 hypertrophy being the result. This deep part of the stratum com- 

 pactum (the only part of it now left) consists of two constituents. 

 The deepest portion of it — that is to say, the portion contiguous to 

 ■the stratum spongiosum — is called the basal layer, and it is very 

 vascular. The superficial portion of the deep part of the stratum 

 -compactum is now transformed into blood - spaces, kno\\'n as the 

 maternal blood-lacunae or intervillous spaces, which receive their 

 blood from the maternal arterioles. The structure, therefore, of the 

 decidua basalis or serotina becomes modified in the latter half of 

 pregnancy, and it consists of the following parts, in order from 

 without inwards: (i) The stratum profundum, unaltered; (2) the 

 stratum spongiosum, the gland-ca\'ities of which have lost their 

 epithelial lining, and have become flattened out ; (3) the basal layer 

 of the stratum compactum ; and (4) the maternal blood-lacunae, or 

 intervillous spaces. 



The basal layer of the stratum compactum is, according to some 

 authorities, composed of synciiium — that is to say, nucleated proto- 

 plasm without any recognisable cells. Vertical septa, composed 

 of syncitium, extend between it and the surface of the chorion 

 between the villi. These septa separate the placental cotyledons 

 from one another, and they traverse the maternal blood-sinuses or 

 intervillous spaces, which are thus broken up into blood-lacunae. 

 These septa contain maternal arterioles and venous radicles. The 

 blood-lacunae, now separated by these septa, are limited, on the 

 maternal side, by the basal laj^er, and, on the foetal side, bv the 



