684 JOHN BEARD, 



proper positions from the very start? Why do they not originate from 

 epithelial cells, as Waldeyer maintained? 



The germ-cells lie at first within the blastoderm, on which an 

 embryo subsequently arises. We have, therefore, to consider the 

 nature of three things, the blastoderm, the embryo, and the germ-cells. 



On various developmental grounds the writer has now for some 

 twelve years or more maintained the blastoderm to be the representative 

 of an asexual generation, or larva, the phorozoon, on and in which the 

 embryo or sexual generation takes its origin. 



From a new direction and in a totally unexpected manner this 

 conclusion acquires fresh support from the facts recorded in the pre- 

 ceding pages. If the blastoderm be not an asexual foundation or re- 

 duced larva, it is really remarkable, even inexplicable, how few of 

 the cells, composing it and contained within it, take part in the 

 formation of an embryo. The latter arises in a very small region at 

 the hinder end of the blastoderm — at the end, indeed, where the 

 cell-material is least abundant! One cannot study the formation of 

 the embryo in longitudinal sections without being convinced of two 

 facts. Firstly, the proliferating zone or "primitive streak" of embryonic 

 formation is at first a very small area, which increases by what, to 

 use a botanical term, might be called interstitial growth. Secondly, 

 apart from this gradually increasing area, enlarged by growth and 

 encroachments at the hinder end, the rest of the blastoderm, origin- 

 ally the greater part of the cell-material, appears to take no share 

 and to have no concern in the formation of the embryo. 



The structures otherwise formed within the former, blood-vessels 

 excepted, never become parts of the embryonic body, and the blasto- 

 derm as a whole in its growth around the yolk-sac in like manner 

 forms structures of an evanescent character. 



Very similar features obtain in development elsewhere. 



There is, perhaps, no region of embryology so deeply explored as 

 Elasmobranch development. As compared with the work done here, 

 from the time of Balfour and Semper onwards until to-day, the 

 development of most other groups of animals has been little studied. 

 More solid service is rendered to science by 10 years of study and 

 work on one animal, or on a restricted group, than in 10 memoirs, 

 published one every year, on as many different animals. It will not 

 be necessary to cite examples of chameleonic and kaleidoscopical 

 research. Illustrations of the first kind of work are afforded by the 

 masterly researches of E. B. Wilson and Eisig in the Annelida. And 



