884 THE PHYSIOLOGY OF REPRODUCTION. 



paternal chromosomes, and thus shares the hereditary characteris- 

 tics of each parent. This -view is represented in a schematic way 

 by Fig. 280, taken from Boveri, the maternal and paternal chromo- 

 somes being indicated by different colors. According to this descrip- 

 tion, both egg and spermatozoon are incomplete cells before fusion. 

 The egg contains a nucleus and a large cell body, cytoplasm, rich in 

 nutritive material, but it lacks a centrosome or the conditions neces- 

 sary for the formation of an astrosphere, so that it cannot mul- 

 tiply. The spermatozoon has also chromatin for a nucleus, and 

 a centrosome or the material which may give rise to a centrosome, 

 but it lacks cytoplasm that is, food material for growth. It 

 would seem that if the spermatozoon could be given a quantity 

 of cytoplasm it would proceed to develop an embryo without the 

 aid of an ovum. This experiment has, in fact, been made by 

 Boveri. Eggs of the sea-urchin were shaken violently so as to 

 break them into fragments. If now a spermatozoon entered 

 one of these fragments, which consisted only of cytoplasm, cell 

 multiplication began and proceeded to the formation of a larva. 

 On the other hand, it would seem to be equally evident that if a 

 centrosome was present in the egg or some influence could be 

 brought to bear upon it to make it form a centrosome it would 

 develop without a spermatozoon. In some animals eggs do nor- 

 mally develop at times without fertilization by a spermatozoon 

 (parthenogenesis), the eggs that have this property probably pre- 

 serving their centrosomes. Loeb* has shown, however, in some 

 most interesting experiments that certain eggs, especially those of 

 the sea-urchin (Strongylocentrotus purpuratus), which normally 

 develop by fertilization with spermatozoa, may be made to de- 

 velop by physicochemical means. His latest method is to treat 

 the egg for a minute or two with an acid (acetic, formic, etc.), 

 which causes the formation of a membrane. They are then placed 

 for a certain interval in a hypertonic sea water, made by adding 

 sodium chlorid to ordinary sea water. They are then transferred 

 to normal sea water and after an hour or so they begin to multiply 

 and eventually develop into normal larva?. Similar although less 

 complete results were obtained previously by Morgan. 



Implantation of the Ovum. After fertilization in the tube the 

 ovum begins to segment and multiply, and meanwhile is carried 

 toward the uterus, probably by the action of the cilia lining the tube. 

 Upon reaching the cavity of the uterus it becomes attached to the 

 mucous membrane, usually in the neighborhood of the fundus. 

 The membrane of the uterus has become much thickened mean- 

 while, and in this condition is known usually as the decidua. The 



* Loeb, "University of California Publications," 2, pp. 83, 89, and 113, 

 1905. See also Wilson, "Archiv f. entwick. Mechanik," 12, 1901. 



