856 PHYSIOLOGY. 



somatopleure becomes so lifted up as eventually to curl up and over 

 the embryo until the fold of one side fuses with that of the other. 

 That is, there is formed the amniotic membrane and cavity. The 

 amnion is a membranous sac consisting of two layers of embryonal 

 cells. The inner layer is composed of ectodermic cells, the outer 

 layer of mesodermic cells. The false amnion, or serosa, comprises 

 all that part of the somatopleure which does not go to form the 

 body-wall and the true amnion. It is also called the primitive 

 chorion and by some authors the chorion. The allantois growing 

 forth from the gut-tract unites with its inner surface and thus gives 

 it vascularity. It is the outermost envelope of the germ. The 

 amniotic sac is filled with a fluid in which floats the foetus. 



The function of the yelk-sac is to furnish nutrition to the 

 embryo for a certain length of time, but is very rudimentary in man. 

 As the yelk-sac disappears by degrees, its place is taken by the 

 allantois. The latter then serves as a medium of nutrition and 

 respiration until the formation of the placenta at the end of the 

 third month. 



Chorion. The chorion is the membrane which envelops the 

 ovum subsequent to the appearance of the amnion. It results from 

 the fusion of the allantois and false amnion. 



Upon the surface of the chorion are numerous villi. At first 

 they are uniform in size, jput at the latter half of the first month 

 there develops an area the villi of which are noted for their long 

 prolongations: the chorion frondosum. This eventually becomes a 

 portion of the placenta. The remaining villi atrophy and finally 

 disappear. 



Chemical Constituents of Spermatozoa. In the head of the 

 spermatozoa of salmon of the Ehine is found a chemical body which 

 is a combination of nucleic acid and a protamin. In different fishes 

 the protamin is given a different name. Thus, we have scrombrin 

 from scromber scrombrius, salmin from salmon, clupein from herring 

 (clupea harengus), sturin from sturgeon (accipenser sturo). The 

 protamins are strong bases and their watery alkaline solutions are 

 intensely akaline, and with acids they form characteristic salts. 

 They all give the biuret reaction, and, it is to be noted, without 

 the addition of an alkali. Protamins are not coagulated by heat, 

 and polarize to the left. These peculiarities place the protamins in 

 a class peculiar to themselves. Clupein, chemically, is identical, 

 according to Kossel, with salmin. 



A peculiarity of the protamins is the high percentage of nitro- 



