338 
cussion, viz, their origin and their distribution. Taking the latter first, 
it may be mentioned that they exhibit a marked tendency to aggreg- 
ation near to and along the blood vessels of the sac. 
As to their origin, I have at present no new facts to record, hold- 
ing it as proved that they mainly, if not entirely, arise during the seg- 
mentation, and as products of that process. With ZIEGLER, AGASSIZ 
and WHITMAN and others, they may be regarded as really cells of 
the hypoblast. 
As to their origin from supernumerary spermatozoa, as first sug- 
gested by RÜückerr!), I cannot, with all respect to the able investig- 
ator, who is responsible for this view, regard it as really proved. 
The nuclei which in Torpedo and Pristiurus have been de- 
scribed by Rtckerr as present in the blastodermic area and just 
outside this prior to the actual commencement of segmentation, cer- 
tainly do exist in this period, as I can myself testify for Raja ra- 
diata. It is an easy matter here to obtain those stages (i. e. at fer- 
tilisation and just after it) in Raja, whose acquisition has given 
RÜckErT so much labour. The fact is mentioned for the benefit of 
others who may feel disposed to undertake a fuller investigation in 
Raja. And I have felt bound to confirm RückErT’s observations for 
Raja, although, before looking into the matter myself, there was a 
strong suspicion in my mind that his observations might bear some 
other interpretation than that he put upon them. The candid con- 
fession may be made, that no other interpretation appears to me pos- 
sible, unless new facts should arise to throw more light upon the matter. 
But, granted that the source of these nuclei be that ascribed to 
them by Rtckert, it by no means follows that their subsequent history 
be the one he suggested. Rickert himself would derive only some 
of the merocytes from them, and as the merocytes exist in other forms, 
for instance Teleostei, in which an origin from supernumerary spermato- 
zoa seems to be out of question (vide AGassiz and WHITMAN, HOFF- 
MANN, ZIEGLER and others) it appears to me to be very hazardous to 
accept the merocytic nature of RÜckeErT’s nuclei. They certainly still 
present a problem for solution, as probably Rückerr himself would be 
the first to admit. 
It may be held as proved, from the researches of those previously 
named, that the merocytes arise in one or more ways as products of 
1) J. Rückerr, Weitere Beiträge zur Keimblattbildung bei Selachiern. 
Mit 1 Tafel. Anat. Anz., Jahrg. IV, p. 353—374. — Zur Befruchtung 
des Selachiereies. Anat. Anz., Jahrg. VI, p. 308—322. 
