20 
EMBRYONIC DEVELOPMENT 
SO). The embryo now elongates, becomes thinner, whilst its each rises (figs. 84, 86 
and 87). Sometimes the anterior region is likewise divided beneath into two halves 
by the prolongation of the longitudinal groove, so that the entire lower portion of the 
body, when seen from below, exhibits four hillock-like elevations (fig. 85). The trans- 
versal groove now deepens and the back rises more and more (figs, 82, 83 and 89), 
until after a few days the embryo escapes, passing into the next period of its history, 
which is that of the larva. 
A single instance of a caudal, needle-like appendage was noticed (fig. 91). But on 
account of the extreme transparency of those appendages, the perpetual and irregular 
movements of the embryos, they may have escaped my notice. 
The internal organization of the hatched embryo is very simple, for nothingisto be 
detected in its interior except large transparent cavities (figs. 91 — 93), enclosed in a 
more dense layer of the enveloping substance, itself surrounded by an external more 
transparent layer. 
XI. 
THE EMBRYONIC SUBSTANCE. 
What is here called embnjonic substance is nothing else but the vitelline substance, 
which, as a whole, became the embryo itself, now moving within the egg’s envelope. 
It is a never resting substance, undergoing the perpetual process of evolution, inces- 
santly renewing itself by the development of its central parts and the decaying of the 
outer parts, in accordance with a physiological principle of the animal fabric. 
We left the vitelline substance when its structure reappeared homogeneous during 
the last stage of the division of the yolk (fig. 45, a). Now, as soon as the embryo re- 
volves (figs. 52 and 53), its substance again is heterogeneous (fig. 54). There are 
very small cells [a, a), in which no nuclei as yet appear to exist ; others, somewhat 
larger (5, b, b, b , b), exhibit very distinctly their nuclei. These nuclei, by expansion, 
become hollow, and appear now themselves like cells, within a mother cell: without 
nuclei first (c, c, c), afterwards become nucleated (d, d ). Some larger cells show 
several nuclei in each ( e , e, e ), the latter growing hollow, transform into new cells 
which, in their turn, become nucleated (g, g, g), showing the third 
generation of cells previously to the bursting of the grandmother cell, which is going 
to take place. A few large oily cells ( h ), are intermingled with these. 
Further onwards, the embryonic substance assumes two aspects (fig. 55). 1st. The 
centre is composed of large cells, some with an oily (c), others with a milky contents 
(b), and still others in which nucleated cells are in the process of formation (a). 2d. 
The periphery or surrounding layer is homogeneous and composed of small nucleated 
cells. Upon the outer surface vibrilke are next formed, originating from the 
peripheric cells themselves (fig. 56). 
