92 ME. LUBBOCK’S ACCOUNT OF THE TWO IMETHODS OE EEPEODUCTION 
The yolk of the agamic eggs of D. pulex at the time of the formation of the blasto- 
dermic layer, breaks up into yolk-masses ‘001 5 in diameter, and containing yolk- 
globules ; in D. Schcefferi these masses are smaller and less evident, but still they are 
l)resent. Zaddach * has noticed the corresponding yolk-masses in Phryganea grandis, but 
asserts that there is nothing similar in Mystaddes ; his observations appear so carefully 
conducted, that it is presumptuous to suppose him to be in error here, but I cannot help 
imagining that in Mystaddes^ as in D. Sdicefferi, the yolk-masses may be present, though 
small and unobtrusive. 
He suggests as a reason for this supposed difference, that the oil-globules are fewer and 
larger in the Phryganea eggs than in those of Mystaddes and it is curious that in 
I), pulex, in which the yolk-masses are larger and more evident than in P. Scheefferi, the 
yolk-globules are larger and fewer. In the eggs of Phryganea^ however, the oil-globules 
are included in the yolk-masses, while the large oil-globule of P. piilex is nearly tuice as 
large as the yolk-masses. In the insects, also, the egg-contents break up into similar 
masses, as may be readily seen in the wasp or the fly ; but this differs from the similar 
process in Phryganea^ in preceding not only the appearance of the embiyo, but also, if I 
am not mistaken, the impregnation. 
When the eggs are first laid, they are spherical, and from 'OOG to ‘01 in diameter, but 
when they have been about two days in the receptacle, they have swollen to -0125 in 
diameter, and they then burst their outer shell or vitelline membrane, and appear as 
broad egg-shaped embryos, consisting of a membranous bag or skin, m which the young 
animal is contained, and which, not having been present when the egg was laid, must 
have been formed since. 
Mr. Huxley has favoured me with the following notice of a corresponding exuHation 
ill Mysis : — “ A blastodermic disk, very similar to that described by Rathke in Astacus, 
constitutes the fii’st trace of the embryo in Mysis. In the middle of this disk a de- 
pression appears, and while the blastodermic disk in front of the depression becomes 
converted into the bilobed rudiment of the head, posteriorly it is produced into a conical 
process directed forwards, and applied against the front part of the blastodermic disk, 
just as is the papillary rudiment of the abdomen and thorax in Astacus., ivith which, in 
fact, it corresponds. 
“ Two papillae, the rudiments of the antennules and antennae, are developed Aom each 
side of the anterior part of the embryo at this stage, and these, as well as the body and 
the bifurcate or fin-like caudal extremity, become invested by a delicate transparent 
membrane. 
“ Up to this point the embryo has been included within the vitelline membrane ; but 
the latter is now burst, and the embryo straightening itself assumes the pear-like form 
described by Rathke and by Feey and Leucea.et, as the first condition of the embryo — 
these observers having overlooked the earlier condition which has just been described, and 
* See his very interesting “ Untersuchungen iiber die Entwickelung und den Ban der Giliederthiere,” 
1st' heft, p. 66. 
