44 



You will learn further that the segmenting egg gives rise, 

 first, to a layer of cells known as epiblast, to which is in 

 due course added another layer, the hypoblast ; and later still 

 a third is added, the mesoblast, these three forming together 

 the gcn)iiiuil layers, so called because they are regarded as 

 containing the germs from which all parts of the embryo will 

 be developed. For instance, the epiblast gives rise to the 

 nervous system, etc.; the mesoblast to the skeleton, etc.; and 

 the hypoblast to the lungs, etc., each layer giving rise to 

 definite structures. These germinal layers must not, however, 

 be confounded with the germ-cells, notwithstanding the fact 

 that it is the mesoblast which is regarded as giving rise to the 

 peritoneum, from which in turn arises the germinal ridges and 

 the primordial germ-cells. Further, you will find reference to 

 certain stages in the development known asblastula,gastrula,etc. 



This te.\t-book teaching many of us have proved for our- 

 selves, for have we not made macro-dissections and micro- 

 sections galore, in which we have found the confirmation we 

 sought ? 



From this brief sketch of text-book teaching, and our own 

 studies, we will turn to Beard's researches. 



In 1888 Beard commenced the researches described in his 

 own words " as undertaken solely with a view of determining 

 " by observation the apparently simple question of the mode of 

 '• development of a vertebrate animal. They had not proceeded 

 " very far before it came to be recognized that whether or not 

 "a 'direct development ' was possible, that presently carried 

 " out in the higher animals had every appearance of being 

 " anything but direct ; indeed, of being based in an antithetic 

 " alternation of generations. This was of such a kind that the 

 " fertilized egg gave rise to an asexual foundation or larva, 

 " termed by me the phorozoon or bearing-animal, and that in 

 " some way or other upon the latter there arose an organism, 

 " termed the embryo, which by reason of its endowment with 

 " sexual organs may be described as the sexual form or 

 " generation.'"" 



♦ Ji)hn Beard, "' A Morphdlogical Continuity of Cjerm-cells as the Basis o( Heredity 

 and Variation." — Rcviexv of Neurology uittl Psychitifry. Jan., 1904 : p. I. 



