No. 2.] THE ORIGIN OF THE SEX-CELLS. 2h3 
found situated at the base of the medusa-bud (Fig. 18). 
Ischikawa speaks of occasionally seeing what he considers 
ova in the ectoderm, and states that he thinks this an 
additional reason for the supposition held by Weismann, that 
the sexual cells do, in fact, come from this layer. It certainly 
seems to point very strongly in this direction, as we could 
imagine that some germ-plasm had failed to pass through the 
supporting lamella into the endoderm, and thus had proceeded 
with its development while in the ectoderm. On the other hand, 
it may simply mean that germ-plasm is present in the ectoderm, 
as well as in the endoderm. 
The male medusa-bud is formed similarly to that of the 
female ; and in Fig. 19, we see an early stage in the development 
of the bud. It is noticeable that in the ectoderm, there are 
numerous nuclei at the apex, a condition of affairs which is 
frequently met with, and has, I find, been noted by Weismann. 
This leads me to believe that the multiplication of the nuclei 
is taking place prior to the formation of the bell nucleus. 
The spermatozoa originate in the ectoderm of the manubrium, 
arranged, as are the ova, in four masses (Pl. I, Fig. 20). A 
small drawing showing the orientation of this manubrium is 
given in Fig. 21. 
It was my desire to follow out the cleavage and further 
development of the egg of Podocoryne, but it seemed impossible 
to keep the medusa alive long enough to obtain their eggs ; 
and then again, the colonies were so few in number, that 
usually but one single colony was found among fifty of 
Hydractinia, and on account of this scarcity, only a male or 
female colony would be obtained at one time. 
In the formation of the medusa the bell nucleus appears as 
a solid mass which pushes the endoderm before it (Fig. 15). 
There is, later, a separation in this ectodermal mass of cells, 
forming an external layer, and an internal mass of cells; this 
mass, also, afterwards, separates into two layers, a middle and 
an inner layer, with a cavity between them. As development 
progresses, the external layer of ectoderm becomes more and 
more attenuated at the oral pole, until finally, both it and the 
middle ectodermal layer break through at this point. she 
