52 



This larval form — phorozoon — referred to by Beard is a 

 far more simple structure, it consists essentially of the 

 blastoderm and yolU-sac. Beard writes : " The blastoderm 

 " grows over and encloses the yollv, and this may be interpreted 

 " as the annexation of the yolk by the phorozoon or larva. 

 " Thence, until the critical period the gametozoon or future 

 " vertebrate is only nourished through the intermediation of 

 " the phorozoon from the yolk."" 



At the critical period, as we have already seen, the game- 

 tozoon commences to suppress the phorozoon, " the parts of the 

 " larva atrophy and are either absorbed, starved to nothingness, 

 " or digested. 



" In those cases where there is no proper yolk-sac, and only 

 " yolk-cells (parts of the phorozoon or larva), these latter from 

 " the critical period onwards break down and degenerate, and 

 " are digested, while in the same instances other parts of the 

 " phorozoon atrophy.""" 



The asexual generation of the vertebrates, like the game- 

 tophytic generation of the higher plants, has been reduced 

 during the process of evolution to comparative insignificance, 

 for, as in the higher plants, the gametophyte is reduced in 

 most instances to but a few cells, so in the vertebrates the 

 asexual form, according to Beard, is represented bj- many of 

 the cells of the cleaved egg of the amphibian, by the blastoderm 

 of the fish and the bird, and by the trophoblast of the mammal, 

 for these various structures he regards as being in the main 

 homologous. 



Having now got some idea of what is meant by the 

 " critical period," and what is to be understood by " larval 

 " forms," we will proceed to consider the origin of the embryo. 

 We have already noted that it is of direct descent from the 

 zygote ; and here too, it may be noted, so also is the larva or 

 phorozoon. The origin of both will be best understood by 

 reference to a diagram drawn up by Beard, bearing in mind 

 his warning that it is a diagram only. 



^ John Beard, " On Certain Prob. Vert. Embryology" ; |-. 51, 

 ♦* Ibid. P. 52, 



