50 



If, then, our old idea with regard to the origin of germ- 

 cells must be given up, may not the generally accepted view 

 concerning the origin of the embryo itself have to be modified ? 

 Assuredly, yes ! We have already noted the view taken by 

 Weismann — that generally accepted — that from the fertilized 

 egg an embryo is formed direct, as shown in diagrams 

 1 and 2. 



It is just this direct development which Beard's work 

 proves cannot take place, as we shall soon note. 

 Seeing that the germ-cells exist before there is any embryo, 

 they cannot possibly be its products. They must, therefore, 

 be direct products of the zygote ; whence then the embryo ? 



Before proceeding to answer this last question, it will 

 be well to ascertain, as clearly as may be, what Beard 

 understands by his " critical period " and his " larval forms." 

 He has written a very lengthy and exhaustive description of 

 the " critical stage " in Scylliiiiit canicida. From it I will 

 take a few extracts which will serve to give a rough idea 

 of what is to be understood ; time would fail for mtjre 

 lengthy detail. Beard writes : — 



" All the organs of the fish were laid down, and 



" histological differentation had begun in all The 



" embryo is rapidly acquiring the adult form of body 



" Finally, apart from many minor details, there remains one 

 " event of far-reaching and fundamental importance to be 

 " recorded. In stating this, 1 should like to be permitted to lay 

 " unwonted emphasis upon its gravity. At this period the embryo 

 " anvexes the contents of the extenuxl yolk-sac, an internal 

 "yolk-sac is formed, and the yolk is gradually drawn into it and 

 " thence into the gut, where, during the rest of tlie development 

 " 'within the egg case, and for some time afterwards, it serves for 

 " the nourishment of the young fish and is digested by the cells of 

 " the gut. Prior to this stage the embryo only obtained 

 " nourishment from the yolk-sac by means of the yojk-sac 

 " circulation, and not even directly through this, but indirectly, 

 "by the intermediation of the ' yolk-hypoblast ' and the 



