1880. | A Sketch of Comparative Embryology. IOI 
The shape of the egg does not necessarily remain spherical, 
but may be altered by external pressure, as when several are laid 
in one capsule (Lumbricus, Nephelis, Planaria, etc.), or when com- 
pressed by an unyielding shell. A very striking instance has 
recently been described by Repiachoff in a 
the Supplement to Vol. xxx of the Zeits- 
chrift für wissenschaftliche Zoologie. He 
describes the egg of Tendra zostericola (a ie 
Sing etilorar tape Figs ga TE T 
One other remarkable modification of the chofi- 
egg-cell occurs among spiders, and has not yet been observed in 
other animals. The eggs of some spiders (Fig. 3) contain, 
besides the nucleus, a second body, 4, of about the same size as 
the nucleus, solid and resistant, and exhibiting indications of a 
Series of concentric laminz; this is probably 
only a specialized form of deutoplasm, similar Le, 
to the four large oil globules described by ont 
Spengel in the eggs of Bonellia viridis, eee 
When an egg-cell attains maturity, the first ps9 
important and striking change that occurs is (2, 
the translation of the nucleus close to the sur- 
face of the egg, where it disappears. The 
nucleus of the ripe ovum is usually called the i | 
germinative vesicle, and the phenomenon just Fic. .3.—Egg-cell of 
alluded to is still generally termed the disap- 7*semerie. domed 
pearance of the germinal vesicle. The fact has cleus; #, laminate 
long been known, but was entirely inexplicable "9% | 
until the discoveries of the last few years afforded a partial expla- 
nation, by rendering it probable that the disappearance is not real 
but only apparent. The subject is still obscure, because the — 
observers are not entirely agreed with one another as to the facts. — 
The greatest difficulty arises from the fact that in most cases the 
€gg-cell ejects two or three small bodies over the spot where 
the nucleus disappears. These bodies are called polar globules, _ 
and are known to occur in Ccelenterates, Echinoderms, Molluscs, 
various classes of worms, Tunicates, Ganoids and mammals, so 
that their existence in all cases might fairly be assumed, were — . 
it not that renewed special search for these bodies in Amphibia, 
by O. Hertwig, had failed to discover any trace of them. No 
