502 THE PHYSIOLOGY OF REPRODUCTION 



syncytium becomes very thin, and occasionally tracts of it are 

 stripped off. 



The decidua serotina, after reaching its full development during 

 the third month, is gradually thinned out. This may be partly due 

 to the stretching of the tissue by the increasing growth of the uterine 

 contents, but it would seem also to depend on conditions of mal- 

 nutrition caused by the blood-stasis (Bonnet), and the choking of 

 the lymphatics by the decidual development (Webster). The 

 resulting degeneration takes the form of a coagulation necrosis or 

 symplasma, as shown by the " Fibrinstreifen," which are comparable 

 to the fibrmous deposits in the rabbit's placenta. The layers of 

 fibrin in the serotina were first described by Nitabuch. 1 They may 

 be seen as early as the sixth week, and even earlier in the reflexa 

 (Webster). They gradually extend more deeply into the substance 

 of the decidua, and also occur in the vessel walls. They are, however, 

 most marked 011 the surface, at or near the junction of the maternal 

 and foetal tissues. That they are due to the influence of the ovum 

 is highly probable from their absence in the vera. Whether the 

 symplasma is formed from the blood or the decidua, or both, is not 

 known. It is probably absorbed by the villous ectoderm during the 

 greater part of pregnancy. 



According to Webster, there may be a new formation of decidual 

 tissue during pregnancy, from irregularly distributed groups of active 

 cells which are present at all periods in the maternal part of the 

 placenta (see p. 402). 



The uterine glands take no part in the formation of the placenta. 

 By the sixth week their superficial parts are largely obliterated, and 

 the deeper parts degenerated. At a later stage, only a few blind 

 ends are seen next the muscular layer. Though their epithelium 

 offers a considerable degree of resistance, and is visible for a long 

 time, its secretory power is probably lost very early. According to 

 Gottschalk, 2 the glandular epithelium undergoes a fatty degeneration, 

 but Bonnet 3 states that the change is a hyaline one. In the vera the 

 glands increase in size and secrete actively for a time. Their secretion 

 is found as a milky fluid in the 'uterine cavity. 4 



1 Nitabuch, "Beitrage zur Kenntnis der menschlichen Placenta," Inaug.- 

 Dissert., Bonn, 1887. 



2 Gottschalk, " Weitere Studien liber die Entwicklung der menschlichen 

 Placenta," Arch. f. (.tyiui'L, vol. xl., 1891. 



3 Bonnet, " Ueber Syncytien, etc.," Monatsschr. f. Geburtslt. v. '/y/w/ - ., 

 vol. xviii., 1903. 



4 Wislocki and Key have studied the distribution and character of mito- 

 chondria (which are sometimes regarded as an index of the metabolic activity 

 of a cell) in the mature placenta of man and other animals. It was found that 

 while mitochondria are present in all placental tissues (ftetal and maternal), 

 they are most abundant in the epithelial cells forming the barrier between 

 the two circulations. They are also abundant in the endothelium lining the 



