800 MR EDWARD J. BLES ON THE 
the outer coat being extremely adhesive. One result of this is that the eggs stick to 
the first foreign body they touch in the water. Another consequence is that the eggs 
become coated with a thin layer of mud when laid in turbid water; the suspended _ 
particles stick to the surface. The appearance of an egg laid in fairly clear water is 
shown in fig. 11, Plate II. Within a few hours the outer layer changes in consistency ; 
it becomes hard and horny, and floating particles no longer stick to it. The horny coat 
is exceedingly tough, and might be regarded as a rudimentary egg-shell. In this 
capacity its function would only be transitory, as on the second day after spawning 
this outer envelope splits and its contents ooze out. The substance of the outer coat is 
so tough and unyielding that the contents are tightly pinched as they issue; the jelly 
and the soft embryo itself are both constricted between the lips of the chink. The ege 
thus freed is composed of a thick outer layer of jelly, the vitellme membrane and the 
elongated embryo lying in the fluid within the vitelline membrane (see fig. 12, Plate II). 
The whole remains adherent to the original place of attachment; the outer coat shrinks 
and forms a shrivelled ring round the place of attachment of the egg to the substratum. 
The escape of the egg-contents only occupies a few minutes. 
A very similar shell-like structure, which seems to be undescribed, is found in the 
egg of Hyla arborea var. meridionalis. Here it is not superficial, but enclosed in a 
thin, soft, adhesive layer which holds together a number of eggs in a clump. Inside this 
layer is a tough, thin, whitish translucent membrane ; then follows a layer of jelly and 
then the vitelline membrane. This “‘egg-shell” is also burst at an early stage. On 
the third or fourth day after the spawn is laid, the spherical shell is found in two hemi- 
spherical pieces lying embedded next to each egg in the jelly of the clump. In this 
ease the split extends meridionally completely round the “shell.” The splitting of this 
membrane in the case of both Xenopus and Hyla is most likely due to the absorption 
of water by the inside jelly and its consequently swelling until the internal pressure 
bursts the non-extensible membrane. This membrane is comparable with the true 
membranous ege-shell discovered by Guppy in the large eggs of Rana opisthodon, 
which are laid in the crevices of rock and enclose the young frog until perfectly formed. 
The Ovum.—The history of the ovum is here taken up at the period of oviposition. 
It then measures 1°5 millimetres in diameter, so that it ranks among the smaller 
Anuran eggs. 
While rotating within the ego-membranes after fertilisation, it can easily be seen 
that the pigmented and unpigmented areas of the surface are approximately equal (see 
fio. 1, Plate I). The pigmented area usually covers rather less than a complete hemi- 
sphere. The dark half is of a rich brown colour, while the yolk is of a very pale 
greenish-blue colour. 
Fig. 1 represents an unfertilised egg about twenty-four hours after oviposition. It 
is easily recognised by the patch of unpigmented protoplasm which has risen with the 
ege-nucleus inside it to the surface, and has displaved the superficial layer of dark 
pigment from the upper pole. This appearance is very characteristic of unfertilised 
