OF PLANOCERA ELLIPTICA. 319 



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 nothingistobe 

 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 embryonic 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, ia accordance with a physiological principle of the animal fabric. 



We left the vitelline substance when its structure reappeared homogeneous durino- 

 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. 64). There are 

 very small cells (a, a), in which no nuclei as yet appear to exist ; others, somewhat 

 larger {b, 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, ff, g), showing the third 

 generation of cells previously to the bursting of the grandmother cell, which is goino- 

 to take place. A few large oily cells (h), are intermingled with the precedino- ones. 

 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 vibrillae are next formed, originating from the 

 peripheric cells themselves (fig. 56). 



I scarcely need allude to^figs. 81 and 82, a as illustrating the permanent activity of 

 the embryonic substance, for they show the same process of cellular life as in fin-, 54. . 

 The proportional size and the degree of development alone differing, hence more im- 

 portant in other respects. 



The substance, in the larval state, is still more heterogeneous than in the embryo, 

 and continues to be such ; there is no return towards that uniform structure of the vi- 

 tellus as observed during its formation, and before and after its division. The future 

 labor of the organism is diversification. 



The substance, in the chrysalis state, I have not been able to examme thoroughly 



