415 
different groups, which is sufficient to illustrate the incor- 
rectness of the system. 
The first stage of development in all Chmtopods appears 
to he very similar, the yelk division has as yet been followed 
out in all its stages in but very few species. Sars in Polynoe, 
Milne- Ed wards in Protula, and Quatrefages in Sabellaria, 
have given great attention to this matter, and the results 
obtained by MM. Claparede and Mecznikow accord very well 
with theirs. In all Chsetopods the division of the yelk pro- 
ceeds so as to produce two kinds of yelk elements, which 
differ chiefly from one another in point of size and con- 
sistency. From the commencement of the process of division 
this distinction takes its rise, for the yelk is at the very first 
divided into two unequal portions. The smaller of these por- 
tions proceeds in its division much more rapidly than the 
larger, so that at length you get a number of small balls or 
cells (the result of yelk division) enclosing larger masses or 
balls The larger enclosed yelk balls are destined for the 
formation of the digestive tract ; the smaller peripheral cells 
serve, on the other hand, for the development of the body 
walls, the muscles, and the nerve system. Thus, in the 
recognised language of embryology we may call the one 
yelk mass the vegetative, the other the animal, layer. In 
this respect we have a very welcome agreement between the 
process of yelk division of Chaetopods and Leeches, the latter 
having been well established by the concurrent observations 
of Grube, Leuckart, and Uobin. With regard to the so-called 
polar cells or Polkugeln, the authors have not much to say. 
Their appearance seems to them to be quite irregular, and in 
many cases they are entirely wanting. The fate of the ger- 
minal vesicle is, the authors observe, a puzzle to them. In 
many species it seemed to disappear entirely upon fertilization, 
and by no method could they bring it again into view. In 
some species, the eggs of which became very dark after ferti- 
lization, there Avas indeed an appearance which is peculiar to 
many Annelids, and which Avas noticed by Quatrefages in 
Sabellaria. But by no contrivance or use of reagents could 
the authors succeed in bringing to vieAv a vesicle among the 
yelk granules. It is noteAvorthy that in thiscase the firstmasses 
resulting from yelk cleavage Avere equally devoid of nuclei. 
In the unmistakable approximation of the Chsetopods to 
the Arthropod-tvpe the absence of a ventral cellular band 
(Keimstreif) has been hitherto a very remarkable fact, since 
this structure is so characteristic for Arthropods. The 
strangeness Avas increased by this, that the presence of a 
ventral band (Bauch-streif) has been Avell established in the 
