ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 38 
that he proves (/. ¢. p. 137) that the division of this cavity accom- 
panies each division of the yelk-mass, and that, eventually, these 
cavities become what he terms the nuclei provided with nucleoli, 
which occupy the centres of the division-masses of the yelk, and 
are the homologues of the embryo-cells of Ascarzs. If both New- 
port’s and Remak’s observations are correct, it would seem impossible 
to deny that the embryo-cells of the Frog proceed from the contents 
of the germinal vesicle. 
I think, then, that considering the only case in which the contents 
of the germinal vesicle are not traceable, under circumstances in whith 
it might be reasonably expected that, if they really exist, they should 
be visible, is that observed by Kolliker ; while, on the other hand, the 
equally definite observations of Nelson, Miller, Gegenbaur, and myself 
(and the less distinct evidence of Newport, Remak, and of Leydig) 
testify to the origin of the blastoderm in one way or the other from 
the contents of the germinal vesicle, in various members of no less 
than four? out of the five primary divisions of the animal kingdom ; 
the balance of the evidence is in favour of the conclusion that the 
embryo-cells are the progeny of another cell, and that here, as else- 
where, extracellular cell-development is a phenomenon of rare, if not 
of altogether questionable, occurrence. 
8,9, 10. Thus far the problem has been to find a parallel for 
those early embryogenetic processes which are ordinarily common 
to large assemblages of living beings. Analogies for the more special 
modifications which the blastoderm undergoes may be sought for in 
the group of which the genus Pyrosoma forms a part. In the first 
place, it may be asked, are there, in this group, any examples of the 
division of the blastoderm into segments, one of which is to serve a 
temporary purpose, while the others become ascidiozooids ? 
Leaving the development of the caudal appendage of ordinary 
Ascidians out of consideration, as hardly a case in point, it yet 
appears that even in these Ascidians, the body of the embryo is, 
during its locomotive stage, divided into two segments, the anterior 
of which gives rise to the so-called suckers (which are diverticula of 
its wall with involuted ends), while the posterior is the rudiment of 
the body. 
Lowig and Kélliker, in their description of the compound larva 
of Botryllus (originally discovered and described by Sars), consider 
the three processes which are given off from the “large round 
1 Celenterata, Mollusca, Annulosa, Vertebrata. J may add, that the first appearance of 
the blastoderm on the surface of the ovisac of Pyrosoma is so like that of the blastoderm in the 
ovum of any of the higher Articz/ata, as strongly to suggest a similarity of origin. 
