1893.] ` Embryology. 59 
outside of the nucleus but close to it there is a body resembling the 
former nucleolus in every particular, except position. This body, the 
metanucleolus, never divides, there is never any radial arrangement 
of the protoplasm about it, and it may be found in one of the blasto- 
meres until a later stage in the cleavage. From a review of the work 
of Metschnikoff, Boveri, and others, the author thinks that homologous 
structures have been seen, although wrongly interpreted, in the Lepto- 
A ntl d apparently also in Mytilus 
and Sagitta. He has also examined Weismann and Ischikawa’s prep- 
arations of the winter eggs of Daphnids, with the result that he 
regards the paracopulation cells as not cells at all, but as in all proba- 
bility structures homologous with the metanucleoli of the meduse. 
Another point of interest in this paper is the numerical relation 
between the chromosomes of the second polar spindle and of the first 
eleavage spindle, there being six in the former and twelve in the latter. 
Boveri had pointed out that in the forms in which this point had been 
studied, while the number varied in different species, the number of 
chromosomes in the cleavage spindle was always just double the number 
in the last polar spindle, and he had also noticed that the number of 
the latter in certain species could be arranged in a geometrical series 
in which the numbers are forms of two (2, 4, 8, 16,32). Hecker 
reviews the literature of the subject and shows that there may be two 
other series besides this one. There is a series of the forms of three 
(3, 9, 27, etc.), of which, however, there is but one example, Echinus 
with nine chromosomes, and then there is a mixed double and triple 
` system (6, 12, 18, 24, 36, 48, etc.), to which Aequoria belongs, as well 
as the greater part of the insects and the vertebrates. He concludes 
that all cases so far known may. be arranged in three systems in such 
a way that in general nearly related forms belong to the same system. 
R. 
meduse, 
IGELOW. 
(1) Zeit. f. Wiss. Zool., 49, 1890, pp. 503-580, plates 24-26. 
(2) Zeit. f. Wiss. Zool., 51, 1891, pp. 685-730, plates 35-37. 
(3) Zeit. f. Wiss. Zool., 54, 1892, pp. 1-249, plates 1-12. 
