REPRODUCTION. 353 



urchin can develop by chemical agents without spermatozoa. He 

 treats the egg for a minute or more with acetic acid, to cause a 

 membrane to form- around it. They are then deposited in a hyper- 

 tonic sea-water, made by the addition of sodium chloride to ordinary 

 sea-water. Afterwards they are transferred to ordinary sea-water, 

 and soon they multiply and develop into normal larvaa. Loeb be- 

 lieves that the unfertilized egg of the sea urchin possesses all the 

 elements for development, and the only reason parthenogenesis in 

 it is prevented is the constitution of the sea-water. Here the pro- 

 cess is mainly ionic. He believes that the nucleus of the sperma- 

 tozoon is not essential, and that it is only a means to stimulate the 

 arrangement of ions surrounding it. 



How does the ovum arrive in the uterus? There is consider- 

 able obscurity on this point. Most observers believe the ovum is 

 discharged into the pelvic cavity, where the cilia of the Fallopian 

 tube propel it toward the uterus. It is in the tube that the sper- 

 matozoon meets the ovum, which here undergoes fecundation, arrives 

 in the uterus, and develops. The spermatozoon is deposited in the 

 vagina or at the mouth of the uterus, and, by means of its cilium or 

 tail, travels up the uterus and Fallopian tube. 



Should the ovum not be impregnated, it dies and passes out of 

 the uterus as a constituent of its secretions. On the other hand, 

 should it become fecundated, the ovum becomes attached to the 

 mucous membrane of the uterus, usually occupying the bottom of 

 some little cleft or pouch. 



The investigations of Peters, of Vienna, and of Webster, of Chi- 

 cago, show that the uterine mucosa does not fold up around the 

 ovum, but that the mucosa at the site of implantation is eroded; so 

 that the ovum eats its way, as it were, into the mucosa, sinking into 

 its depths until the edge of the swollen mucosa closes over it, thus 

 forming the decidua reflexa. 



The position to which the fecundated egg becomes attached is 

 the decidua serotina, and it eventually forms the placenta, the 

 nutrient organ of the embryo. Before the ovum arrives in the 

 uterus it has formed the amnion and chorion with the villi of the 

 chorion. Some of the ectodermal cells in the chorion become spe- 

 cialized to form what is called the trophoblast, and this probably 

 transfers nourishment from the mother to the ovum. 



After its formation the mesoderm grows by reason of its own 

 cell-proliferation, and is independent of its dual source. Along 

 either side of the median line the mesoderm presents a thickening 



