ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 709 



at work indirect causes wliicli are equivalent to a want of the indi- 

 viduals of one sex. 



In another sense the influence of nutriment is of the greatest 

 importance ; notwithstanding the fact that a starving animal may 

 reproduce itself numerously, the progeny are feebler than those of 

 one which only produces as many as, under these conditions, can live 

 and thrive. Surplus nutriment leads to the production of a stronger, 

 and insufficient food to that of a weaker progeny. Domesticated 

 animals breed earlier, and are more fruitful than the wild ; townsmen 

 and sedentary people more than countrymen who take much hard 

 exercise. More children are conceived in summer than in winter ; 

 and in Scotland, according to Haycraft, the maximum of conceptive 

 capacity is simultaneous with that of the thermometer. Birds, bats, 

 and insects breed less numerously than fishes, and especially than 

 parasites who use up little or no force in movement. 



The differences we observe between males and females are easily 

 explained when we consider the very different parts that they play in 

 the physiology of reproduction ; it must always be remembered 

 that it is the office of the female to produce the material out of which 

 the embryo is built up. 



The author concludes by reasserting his conviction that it is in 

 the principle of natural selection that we must look for the exj)lana- 

 tion of the differences between the sexes. 



Rudimentary Placenta in Birds.* — One of the principal dis- 

 tinctions between the Mammalia and the lower Vertebrata has been 

 hitherto supposed to be the possession by the former of a placenta. 

 M. Duval has, however, come to the conclusion that this structure is 

 not exclusively confined to the Mammalia, but that it also exists, 

 though in a rudimentary form, in birds. 



The allantois in passing inwards into the pleuro-peritoneal cavity 

 does not become attached to the amnion or to the umbilical vesicle, 

 but joins the chorion, becoming fused with it ; it ends by forming 

 a sac which incloses a mass of albumen ; into this sac the villi of 

 the chorion project, and an organ is thus formed which is completely 

 analogous to the placenta of the Mammalia ; the different form of 

 the organ in birds and in mammals is evidently owing to the dif- 

 ference between the oviparous and viviparous method of reproduction ; 

 the villi of the chorion in Mammalia are attached to the body of the 

 mother, while in birds the necessities of the case demand that they 

 should be developed upon the opposite side of the chorion and attach 

 themselves to the nutritive albumen. It is, however, quite intelli- 

 gible that in an ovoviviparous vertebrate, whore the egg has a thin 

 membranous shell, the placontoid organ should become attached 

 to the internal surface of the oviduct. This placenta of birds is 

 therefore a rudimentary organ which enables us to understand liow 

 the placenta of the Mammalia may liavc originated. A jjhysiological 

 difference between the placenta of birds and of Mammalia is that in 



* Joum. Anat. ct Physiol., xx. (1884) pp. 193-201 (4 pis.). Soo also this 

 Journal, ante, p. 3G0. 



