686 JOHN BEARD 



but one or two original cells, what he here writes might be applied word 

 for word to the formation of the Elasmobranch embryo on a blastoderm. 

 But, lest the writer should be supposed to see this only in his own 

 imagination, let it be added, that the sudden change in the direction 

 of growth and cell-division at the close of segmentation was com- 

 mented upon some years ago by R. Assheton ^) in the development of 

 the frog. 



The gist of the whole matter is, that the primary product of the 

 segmentation of Maja is the foundation (Anlage) of an asexual gener- 

 ation or phorozoon, this containing, also as products of the cleavage, 

 a certain number of primary germ-cells. This foundation with its 

 contained germ-cells is the morphological equivalent of the hydroid 

 polype of a Hydrozoon. It differs from this latter, apart from points 

 of anatomy, in being unable to increase by budding, and in giving 

 rise to but one "sexual person" or "embryo". 



Occasionally, of course as an abnormality, or as a more or less 

 perfect twin, two such embryos may form upon it. Often these do not 

 get beyond the "primitive streak" phase. Cases of this kind have 

 been recorded by Spencer in the chick, by Assheton in the sheep, 

 and by others. It has already been stated, that the "embryomas" 

 of Wilms are embryologically to be looked upon as such abortive twins. 



As VON Jhering^) long ago pointed out, the generally accepted 

 idea, that the origin of but one single embryo from an egg is the 

 normal and primitive (ursprüngliche) condition, is the opposite of the 

 truth. He insisted, that in fact the origin of several embryos from 

 one egg must have been the original condition (just as numbers of 

 medusae with their cargoes of germ-cells take their birth from one 

 asexual stock, itself the product of one fertilised egg). For one of 

 the Edentata, Praopus hyhridus, he proved this to be the case. Here 

 he found, that several embryos, each with its own amnion, occupied 

 one common chorion, and that all the animals were of the like sex, 

 either male or female. But for the latter circumstance and but for 

 the fact, that abortive germs of early periods also obtained in the one 

 chorion, the observation might possibly bear the interpretation, that 

 the foetal membranes had fused together to form one chorion. 



1) Assheton, R., The growth in the length of the frog embryo, 

 in: Quart. J. microsc. Sc, V. 37, p. 223—243. 



2) VON Jhbring, Heemann, Ueber "Generationswechsel" bei Säuge- 

 thieren, in: Biol. CtrbL, V. 6, p. 532—539, 1886. 



