1887] e Embryology. 93 
five hundred and eighty-nine forms occur in the Amazons valley, 
twenty-seven and one-half per cent. in Venezuela, thirty-three 
per cent. in Columbia, thirty-six and one-half per cent. in Ecua- 
dor, forty-seven and one-half per cent. in Peru, thirty-three per 
cent. in Southeast and Central Brazil. The West Indies have but 
four per cent. of the birds of Guiana, or no more than are pos- 
sessed by the Argentine Republic. 
EMBRYOLOGY.: 
The Formation of the Eggs and Development of Rotifers.? 
—G. Tessin has made a very important contribution to the life- 
history of the wheel-animalcules, which he has traced in Brachi- 
onus urceolaris, Euchlanis dilatata, Salpina mucronata, and Rotifer 
vulgaris, having succeeded in obtaining satisfactory sections of the 
embryos in a number of stages. 
The large simple sac opening into the cloaca, which has hith- 
erto beén regarded as the ovarium, is, according to Tessin, not 
an ovary at all, but the eggs are developed on the outside of this 
organ from a heap of cells lying on its right side and near its 
anterior end. As a rule, the number of nuclei in the ovarian 
mass is constant, eight nuclei being the usual number; only in 
the fixed Tubicolariz, Philodine, and Pterodina could a larger or 
smaller number of ovarian nuclei be made out. 
In the process of maturation the nucleus of the egg gradually 
passes to the periphery, where it breaks up; but before it does 
so a nuclear spindle is developed. This process Tessin regards 
as an indication that polar cells are extruded, although he did 
not actually succeed in finding them. 
None of the accounts hitherto given of the manner of segmenta- 
tion are correct, according to this author. The egg is first divided 
transversely into two unequal cells, the cleavage plane being 
also slightly oblique, and the larger cell anterior, the smaller 
t Edited by Dr. JoHN A. RYDER, Philadelphia. 
Ueber Eibildung und Entwickelung der Rotatorien, Zeitschr. f. Wiss. Zoologie, 
xliv., 1886, pp. 272-302, pls. xix., xx. 
