336 PHYSIOLOGY. 



cicatrix formed; it is a horse-shoe -shaped, bent, but longer longitudinal 

 projection, which lies upon one side of the egg, but which is yet 

 observed only in a few eggs, for instance, in Phasma. Its pur- 

 pose is not yet ascertained, although probably it is the analogue of the 

 tread, and consequently thence the development of the embryo would 

 originate. During this period the placentula retains tolerably long its 

 former conical figure, but it loosens and becomes lighter as a distinct 

 proof that it has lost something (the imbibed impregnating semen?), 

 but henceforward it decreases with the increase of the shell and, 

 pellicle beneath it, and, at last, entirely disappears when the develop- 

 ment of the egg is completed. This, after the formation of the shell, 

 is limited to involution, and yet, at least in Phasma, a new structure 

 is added to it, namely, a crown-shaped appendage at the end of the 

 egg, in direction from the egg duct. This crown, which is formed of a 

 hard horny trellis-work, and which at its apex has a round aperture, 

 rests upon a correspondingly large orbicular depression in the shell ; 

 at this spot also the pellicle appears more delicate than elsewhere. 

 Beneath it is found a small vacant space, into which, the tracheae which 

 during the formation of the embryo, are forming in the vascular 

 membrane, together with their main stem, open themselves. This 

 delicate membrane may therefore justly be called the egg gill, for 

 through it the air passes into the egg. In those eggs which have no 

 crown, as is the case with the majority with which we are acquainted, 

 the orbicular depression is very small, but it lies likewise at the end 

 (PI. I. f. 23.). The indicated involution of the egg has chiefly reference 

 to the yolk, which has not. yet completely filled the shell, it conse- 

 quently appears, as well as the pellicle which closely envelopes it, 

 folded upon the surface ; but it acquires consistency, and exhibits cells 

 in which, particularly towards its circumference in Phasma, a purple- 

 coloured mass is deposited, whereas in other cases it is yellow or 

 greenish. The more the yolk increases, the faster the folds disappear, 

 and when the egg has acquired the maturity requisite for being laid, it 

 entirely fills the shell, with the exception of the small vacant space 

 beneath the germen. During this period of ripening the inner tunic 

 of the egg-tube separates closely above the upper end of the egg, and 

 dissolves into a pappy consistence, which is excluded together with 

 the matured egg. The inner membrane with the next egg then 

 descends to the base of the egg tube, and the development of the new, 

 now lowest, egg germ proceeds in the same way. 



