426 
W, A. HASWKLL. 
here and there embedded in the yolk, at a little distance from 
the main body. 
In the next stage observed this posterior elongation of the 
blastoderm has reached the ventral surface, in what is destined 
to be the genital region, and has become almost completely 
separated off from the anterior part, in which the further 
development of the brain, the pharynx, and the exci’etory 
system is going on. There thus come to be two distinct foci 
of development, an anterior and a posterior. From both 
superficial cells must be separated off to form the epidermis, 
since this layer is to be recognised as a distinct, though very 
thin layer, with widely separated nuclei, at the stages when 
the tentacles and sucker first begin to be formed. This layer 
is early completed on the ventral surface, but on the dorsal 
surface it does not make its appearance till a considerably 
later stage. 
The rudiments of the tentacles (PI. 24, fig. 11) make their 
appearance at a stage when certain changes in the cavity 
have led to the first differentiation of the pharynx, and when 
the rudiments of the eyes have first become distinguishable. 
They appear as processes which grow at first straight for- 
wards from the anterior extremity of the body; but as they 
elongate, they become bent (PI. 25, figs. 12, 13), usually 
towards the ventral, but in many cases towards the dorsal, 
side. In the former case the cephalic portion of the body 
(PI. 25, fig’. 13) becomes strongly flexed ventrally at the same 
time. When this takes place the part of the surface covered 
by the reflexed tentacles develops a system of minute tooth- 
like epidermal papillm, which are apparently of firm con- 
sistency. 
The rudiment of the sucker makes its appearance about the 
same time as those of the tentacles. Epidermal papillae, 
similar to those underneath the tentacles, are formed under 
the sucker. 
We have left the endoccfile as an extensive rounded space, 
occupying nearly a third of the length of the egg, with a 
floor and a roof. As the cavity reaches its greatest dimen- 
