AND DEVELOPMENT OF MYRIOTHELA. 
563 
They are more minute than the spermatozoa of any other hydroid with which I am 
acquainted. 
By what means the spermatozoa naturally escape from the gonophore I have not been 
able to determine with certainty. I could find no external orifice, nor could I detect a 
thinning of the summit of the gonophore like that which in the female precedes the 
escape of its contents ; and when the mature male gonophore was subjected to pressure 
it was always by the rupture of the spadix and the escape of the spermatozoa through 
the peduncle, which would thus carry them into the cavity of the blastostyle, that the 
gonophore became emptied. It is not improbable, as we shall afterwards see, that this 
is their natural mode of escape. 
Returning now to the contents of the female gonophore which, just after their 
escape, we had left in the grasp of the claspers, we find that by this time the coalescence 
of the separate plasma masses into a single spherical plasmodium has been completed ; 
and it is probable that fecundation now takes place. Hitherto we have seen nothing 
which can be compared to any phenomena which we would be justified in regarding as 
the immediate consequence of the action of the male element on the female ; but soon 
after the liberation of the pl’asmodium and its seizure by the claspers, we find that the 
whole has become broken up into a multitude of small round or irregularly shaped 
masses (Plate 57. fig. 17). Some of these maybe seen still connected to one another by 
narrow isthmuses of their substance, while others are quite free, and can be isolated 
under the microscope. They all consist of a granular protoplasmic matter without any 
distinct boundary membrane, and with numerous nucleus-like bodies immersed in their 
substance. The common external structureless membrane is distinct, but it is still thin 
and weak. 
I must regard this breaking up of the plasmodium into separate masses as repre- 
senting a true segmentation, such as in the simple ovum occurs as the immediate result 
of fecundation. I have not, however, succeeded in witnessing its earlier stages, and I 
cannot say whether it proceeds in accordance with the ordinary binary law of vitelline 
segmentation. 
How far this breaking up of the plasmodium is continued before a true histological 
differentiation becomes apparent, I am unable to say, for the next stage which showed 
itself (Plate 58. fig. 1) presented a marked advance on the previous ones. The seg- 
mented condition had now entirely disappeared, and the developing mass had acquired 
a true cellular structure, while it had become further differentiated into two distinct 
layers — an external (a) layer, ectoderm, in which the cell-boundaries were with some 
difficulty made out, and an internal (b), endoderm, composed of very obvious cells larger 
than those of the ectoderm, and each with a clear nucleus and granular protoplasm. 
This internal layer formed the boundary of a cavity ( c ) produced apparently by lique- 
faction of the more central parts of the mass. 
The developmental stage to which we have now arrived is thus represented by a 
hollow spherical body, whose walls are formed by two layers, an ectoderm and an endo- 
