1895.] Embryology. 945 
EMBRYOLOGY: 
Eggs of Nematodes.—Hans Spemann contributes to the May 
number of the Zodlogische Jahrbücher an elaborately illustrated ac- 
count of the cleavage of the eggs of the Nematode Strongylus para- 
dozus. In general it is a confirmation of the results obtained by 
Boveri upon Ascaris megalocep hala. 
The egg divides into two equal cells, yet one contains all the yolk. 
Each divides into two'and the four so produced become rearranged in a 
characteristic way. 
Tho two cells from the one containing no yolk divide into right and 
left cells that increase to form the major part of the ectoderm at the 
period of gastrulation. One of the other two cells gives rise at its first 
division to entoderm and mesoderm, while the other produces four, of 
which three add themselves to the ectoderm and one remains as the 
originator of the sexual cells. 
The author compares this cleavage to the divisions of an apical cell 
in a plant; the egg divides off an entoderm cell, a mes-entoderm cell 
and ectoderm cell, another ectoderm cell and finally remains as the 
origin of the sexual cells. The sexual cells may be thus readily traced 
backed to their ancestors amongst the blastomeres. They are sepa- 
rated as special cells in the fourth generation, starting from the undi- 
vided egg. 
In this process of rapid separation of sexual and somatic cells, 
Boveri found in Ascaris megalocephala a peculiar nuclear differentia- 
tion. At the first cleavage the nucleus of one cell looses part of its 
chromatin and its chromosomes undergo a change of shape. The 
other cell undergoes a like change when divided, and so on till after 
five divisions all the cells but one have the modified nuclei. This cell 
with the unchanged nucleus becomes the the beginning of the sexual 
cells. i 
This remarkable nucleus has been sought for by Oscar 
Meyer in the eggs of other nematodes namely, Ascaris lumbricoides, A. 
rubicunda, A. labiata, A. mystax, A. perspicillum, Strongylus tetracan- 
thus, S. paradoxus and Oxyuris vermicularis. In the first three he 
finds essentially the same process as in the species studied by Boveri, 
1 Edited by E. A. Andrews, Baltimore, Md., to whom abstracts, reviews and 
preliminary notes may be sent. : 
2 Jenaische Zeitschrift., 29, May 15, 1895. 
3: g ‘oe ee 
