51 



" specialized parts of this, the merocytes. Taking the whole 

 " of their characters into account, the embryo is now for 

 " the first time a young fish, and it is independent of its 

 " transitory, or larval, or asexual foundation. It can set about 

 " feeding itself, and is powerful enough to begin the task of 

 " suppressing the transient foundation, including the transient 

 " nervous apparatus, the merocytes and yolk-hypoblast, and 

 " other evanescent structures." '■' 



This "critical period" holds good for mammals also, so 

 far as they have been studied. In certain Marsupials it is 

 coincident with the epoch of birth — the then immature con- 

 dition being the epoch referred to. 



In the higher animals — sheep, rabbit, man, etc. — it is 

 certain that the allantoic placenta is an embrj'onic organ formed 

 just prior to the " critical period " and which now commences to 

 supersede or replace the ectoplacenta, or trophoblast, or in the 

 words of Heisler : " The development of the allantois and its 

 " accompanying system of blood vessels is simultaneous with 

 " the decline of the yolk-sac and the vitelline circulation.""" 

 This in man begins to take place during the sixth week of 

 gestation, in the rabbit on the 15th day, and in the sheep at 

 the 30th day. 



The point to notice here is that this ectoplacenta or 

 trophoblast which is replaced by the allantoic placenta is 

 regarded by Beard as the larval structure — phorozoon — asexual 

 generation. This larval form must not, however, be re- 

 garded as the equivalent of the caterpillar or the tadpole ; 

 these must not be regarded as larval forms at all, for 

 as Professor Miall first pointed out — these are really adult 

 forms, and their transformation into imago and frog is a 

 secondary transformation, connected with the development and 

 maturation of the sexual organs. 



■ John Beard. " On Certain Problems of Vertebrate Anatomy." Published by 

 Gustav Fischer. Jena. 1896. 



♦♦ Heisler. ■■ Text-Hook of Kmbryology," p. 148. 



