SECRETION OF MILK 107 



is provided with superficial muscles whose con- 

 traction must serve to squeeze the milk out. 

 These vascular villi give a shaggy appearance 

 to the mucous membrane of the oviduct, and 

 Alcock describes them as "dripping with milk" 

 and discharging a creamy, albuminous fluid. 1 This 

 method of intra-uterine nutrition of the young by 

 a special secretion of the uterine glands offers at 

 once a great contrast and a remarkable parallel 

 to the corresponding phenomena in mammals. 



Every system of organs throughout the animal 

 kingdom will be found to yield abundant instances 

 of convergence. An excellent example of what 

 may be called gastral convergence is exhibited 

 in the structure of the gizzard of some Teleostean 

 fishes. In the first place, as between the pyloric 

 gizzard of the few fishes which possess one and 

 the pyloric gizzard which is so characteristic of 

 birds, it is to be noted that the most absolute 

 comparability prevails, differences in detail not- 

 withstanding. They are homoplastic modifica- 

 tions of a homologous structure, namely, the 

 pyloric division of the vertebrate stomach. The 

 exceptional occurrence of a gizzard in fishes 2 and 

 the regular presence of one in birds give no more 

 indications of affinity than does the gizzard of 



1 A. Alcock, "A Naturalist in Indian Seas," London, 1902, 

 see pp. 71, 159, 210. This valuable book contains a bibliography 

 of the published work of the Indian Marine Survey ship In- 

 vestigator. 



2 Compare also the gizzard of the toothless Ant-eaters. 



