DEVELOPMENT OF SIMOCEPHALUS 633 
They are several times as large as those of the blastoderm 
cells, as will be seen from fig. 8. The chromatin in them is 
either very scattered or very scanty. Each nucleus contains 
several nucleoli which stain to varying degrees, but none stain 
at all deeply. These cells are the primordium of the gonads. 
Commencing at the earliest stages when the nuclei of the 
cells of the ventral mass are still obscure, cells can be seen 
round the posterior periphery which are passing inwards and 
dorsally up the inside of the blastoderm. These cells are 
apparently formed by proliferation of the blastoderm cells 
round the edges of the ventral mass and then migrate inwards 
at its periphery. They are mesoderm cells and will be spoken 
of as the Ectomesoderm. Later their nuclei become more 
distinct and are seen to be larger than those of the blastoderm 
and to contain distinct deeply staining nucleoli. 
Soon after the genital rudiment becomes distinct there 
appear on the dorsal side of the embryo the primordia of the 
nervous system—the ‘Scheitelplatten’. These consist of 
two groups of tall columnar cells symmetrically placed about 
the median plane, in which the nuclei are large and oval, 
approximately twice as long as the nuclei of the neighbouring 
blastoderm cells. The nucleoli are deeply staining and very 
conspicuous, and there is a marked absence of chromatin in 
the remainder of the nucleus. They agree with those described 
by other workers on Cladocera, and their further development 
will not be treated here. 
A very conspicuous change is now brought about in the 
embryo by the invagination of the genital rudiment. An 
early indication of this inward migration can be seen in fig. 38, 
where the surrounding cells are seen to be pushing their way 
over the primordial germ-cells. The primitive germ-cells sink 
into the egg, a variable but sometimes considerable distance. 
The pit caused by this sinking in has been seen to stretch 
a third of the way across the embryo. The lips of this pit are 
formed of the ectomescdermal cells which are continually 
pushing their way under the edge of the genital rudiment 
to lie on the inside of the blastoderm (fig. 4), and as the 
