486 EMBRYOLOGY OF THE TURTLE. Part III. 
of this membrane, the primitive egg-cell wall, and have here only to speak of its 
permanence up to a certain period, (Pl. 9a, fig. 18, 18a, ¢,) when the egg has 
attained to a size of about one tenth of an inch in diameter, beyond which, very 
probably, its function is supplied by the already well developed zona pellucida, 
whilst it is gradually resorbed; at least, we have observed no trace of its existence 
after this time. Of its changes, from the time of its origin till its disappearance, 
little can be said of such a thin, apparently structureless membrane, beyond the 
mere notice of the gradual thickening and defining of its outline, till there 
is no doubt left of its perfect distinctness, as a layer, from the yolk which it 
incloses.! 
The Embryonal Membrane? On the surface of the yolk, in an egg hardly visible 
to the naked eye, there is a layer of minute, smgly mesoblasted cells, (Pl. 8, fig. 
12, d; Pl. 9a, fig. 13, d,) apparently not yet connected with each other. On account 
of their size, and of their color, which resembles that of the yolk in the intermediate 
neighborhood, it is no easy matter to recognize these cells at first sight; but when 
once seen, and their peculiarity noted, their presence may afterwards be readily 
detected by a practised eye. 
but one explanation, which is, that they are peculiarly modified yolk cells. 
As to the mode of their origin, there appears to be 
In con- 
firmation of this explanation we may mention their outline, which as yet is thick, 
is now called vitelline sac is the primitive cell wall 
of the primitive ovarian egg. 
1 Thompson (article Ovwm in Cyclop. Anat. p. 78) 
compares the early yolk sac of Birds (which he 
hardly admits as a true membrana vitelli, notwith- 
standing Meckel’s researches) to the zona pellucida 
of Mammals, (the true primary vitelline sae of these 
animals, interior to the zona, being totally ignored by 
him; see also p. 50, where he describes the zona as 
the original yolk sac, and the only one existing in 
Mammals,) and the secondary yolk sac (the true zona) 
to the tunica granulosa of viviparous Vertebrates. 
The secondary yolk sac, he infers, is derived from the 
cellular lining of the Graafian follicle ; but, since at 
the same time he makes it merely the exterior stratum 
of a concentric series, the inner of which, he insists, 
become the true yellow yolk granules, (the primary 
yolk sac, zona pellucida, as he calls it, having disap- 
peared by deliquescence,) it looks very much as if he 
had mistaken the development of the “ membrana in- 
yestiens” for that of the membrana vitelli. Again 
he says, (p. 78,) “the external edge of the layer of 
prismatic cells, the length of which is considerably 
increased, is now surrounded by a narrow, pellucid 
space inclosed by a double line, presenting the ap- 
pearance as if a small part of the bases of these cells 
had been fused together in a homogeneous film.” 
This, probably, is the true zona pellucida of Birds; he 
having failed to see the membrana vitelli, (already 
disappeared, as he thinks,) situated between it and 
the layer of prismatic cells, from which latter he sup- 
poses, but without direct research, that the “ pellucid 
space,” because of its traces of hexagonal markings, 
is an immediate development. 
? Until more extensive investigations have proved 
the identity of this membrane with the “ Keimblase ” 
of Bischoff, or the “ Umhiillungshaut” of Reichert, 
which is called “investing membrane” by some Eng- 
lish writers, it seems best, in order to avoid confusion, 
to give it a distinct name. That of “embryonal mem- 
brane” appears the most acceptable and significant. 
It can hardly be an objection that it recalls the em- 
bryonic envelopes, for it is in the end more or less 
intimately connected with them. 
