682 JOHN BEARD, 



ception of the germ-cells as unicellular organims is totally at variance 

 with any revelation of their "phylogeny" in the development. 



It is, therefore, not from ignorance of the trend of embryological 

 opinion, but from a strong and decided objection to undertake a futile 

 and impossible task, that this fairy-tale of the germ-cells is a blank 

 in the present writing. 



But to my mind there is another chapter of their history full of 

 absorbing interest. It has already been stated, that the germ -cells 

 are formed outside the embryo; indeed, that they are present before 

 there is any embryo, and that they migrate into it after it has begun 

 to unfold. Out of what do they wander? Out of a flattened 

 blastoderm, which some thirty-five years ago the illustrious founder 

 of the science of Comparative Embryology, Carl Ernst von Baer, 

 likened to the flattened or spread-out larva of an Echinus. He writes 

 in a memoir of great import, but with an unfortunate title, as follows : 

 "Man könnte dies Blastoderma des Vogels ebenso gut einen Larven- 

 zustand nennen, denn dass die Larve des Echinus wie ein halbaufge- 

 schlagener Parapluie aussieht, kann bei so weit abstehenden Thieren 

 kein Einwurf sein gegen die Bemerkung, dass auch die Wirbel thiere, 

 obgleich in übereinstimmender Reihe embryonaler Entwicklungen fort- 

 schreitend, doch im ersten Anfang sehr verschieden erscheinen. Das 

 Blastoderm liegt schlalf auf dem Dotter. In den Echinoderraen wird 

 es durch feste Stäbchen offen gehalten, etc."^). 



This passage, like many others in the same memoir, proves that 

 von Baer had at least an inkling of what I have termed "develop- 

 ment by substitution of organisms ", or antithetic alternation of 

 generations. 



Throughout the course of the present research the writer has 

 endeavoured to the best of his ability to exclude any and every theo- 

 retical consideration; but, surveying his gleanings at the very close 

 of the work, he cannot overlook, or neglect, the obvious bearings of 

 some of the finds. 



The facts, recorded in the preceding pages concerning the wander- 

 ings of the germ-cells of Raja^ strikingly resemble in their general 



1) VON Baer, Carl Ernst, Ueber Prof. Nic. Wagner's Entdeckung 

 von Larven, die sich fortpflanzen, Herrn Ganin's verwandte und er- 

 gänzende Beobachtungen, und über die Paedogenesis überhaupt, in: 

 Bull. Acad. Sc. St. Pétersbourg, V. 9, p. 203—308, 1 plate, 1865. One 

 of the most learned papers on general embryologj^ ever written, of im- 

 mense importance to every student of the science. 



