6 NATURAL SCIENCE. January. 



generation. Thus in a dogfish at the critical stage the embryo is 

 definitely a dogfish, with signs of the peculiar snout, paired and 

 unpaired fins. Up to this stage the nutrition has been through the 

 medium of special cells of the external yolk-sack which have emulsified, 

 and, so to speak, made pap of the yolk. After this stage, the yolk is 

 gradually drawn up into a new and internal yolk-sack, formed from 

 tissue which becomes an actual part of the future fish ; and the yolk 

 is digested directly by the alimentary canal of the future fish, the 

 pancreas of which has begun to secrete. The external yolk-sack 

 disappears. A corresponding stage Mr. Beard thinks to have made 

 out, or at the least to have pointed to, in all the vertebrate groups. 

 In the case of opossums and marsupials without the allantoic placenta 

 described by Mr. Hill, the embryo is born at the critical stage, and, 

 as the yolk-sack of these and higher animals is already devoid of yolk, 

 the change in nutrition is not from external to internal yolk-feeding, 

 but to the new diet of milk. Mr. Beard considers that the milk-glands 

 are older than the true placenta, and that their coming into play 

 marks the change in the nature of the embryo. In animals with an 

 allantoic placenta the critical stage is marked, not by birth, but by 

 change from yolk-sack placenta, such as is found in the marsupials, 

 to the allantoic placenta, which is an outgrowth from the true embryo, 

 not from the yolk-sack. As the allantoic placenta became more 

 important, the sphere of the mammae became less important, and 

 indeed Mr. Beard looks to the ultimate disappearance of suckling. 

 Thus, in some ungulates the young are nourished so long in the 

 uterus that a few hours after birth they are able to feed on grass. As 

 having a possible bearing in favour of his view that nutrition by milk 

 is older than nutrition by a true placenta, we may remind him that 

 one of the earliest signs of pregnancy in woman is change and activity 

 in the milk-glands. 



At the critical stage in all vertebrate embryos, a similar degree of 

 development has been attained : the central canal of the nervous 

 system has begun to form, constriction of the notochord has begun, 

 the sex has been determined, and the permanent kidney has been 

 formed. But where Mr. Beard charges against the theory of recapitu- 

 lation is this : at the critical stage the final characters of the adult 

 are sufficiently marked to determine the young embryos as respec- 

 tively fish, amphibian, reptile, bird, or mammal. Indeed, Mr. Beard 

 goes further, and declares that in the case of the human embryo the 

 critical stage is definitely human. To put the difficulty plainly : in 

 its crudest form the recapitulation theory supposes that the 

 mammalian embryo passes through a fish-like stage when the gill- 

 slits are developed. Mr. Beard declares that before the gill-slits have 

 appeared the embryo has already assumed definite mammalian 

 characters : that it becomes a mammal before it becomes a fish. 



It would be unfair and unwise to criticise Mr. Beard's con- 

 clusions when we have found too little space for his arguments ; but 



