418 
EDWIN G. CONKLTN 
the endoderm and ectoderm now takes place, and is plainly shown be- 
fore the commencement of the next period of activity which is to result 
in the separation of the macromeres from the micromeres. In a side 
view of the egg at the end of the second period of rest, fig. 11, each 
spherule is divided into three pretty well marked regions, distingi ished 
from each other by the amount of deuteroplasm contained in the pro- 
toplasm. The formative end of each spherule is quite transparent and 
contains none of the large spherules of deuteroplasm, and only a few 
granules. The central portion of each spherule is quite granular and 
contains many of the food spherules, but it is much more transparent 
than the yolk of the unsegmented egg. A large nucleus is also very con- 
spicuous in this region. Almost all of the food material is massed in 
an opaque ball at the nutritive pole of each spherule. 
In subsequent stages he observed the formation of two sets of 
micromeres, containing the transparent substance at the forma- 
tive pole, also the macromeres composed largely of food sub- 
stance, and he determined that the ectoderm came from the 
former and endoderm from the latter. 
The year in which this paper by Professor Brooks appeared 
also witnessed the publication of Rabl's (1879) beautiful and ex- 
tensive work on the development of Planorbis. He observed 
that the protoplasm of the freshly laid egg showed a significant 
polar differentiation, the animal half being composed of small 
and the vegetative half of large yolk granules; in reflected light 
the former appears whitish, the latter yellow. He followed the 
cleavage in detail and showed that at the 24-cell stage there exists 
a complete separation of the three kinds of elements out of which 
the germ layers take their origin; and after showing that this dis- 
tinction in these elements is recognizable in the 8-cell, the 4-cell 
and the 2-cell stages, he says (p. 572) : 
Unwilkiirlich drangt sich uns nun die Frage auf, ob wir nicht auch 
schon in der ungefurchten Eizelle eine ganz bestimmte und gesetzmas- 
sige Anordnung und Vertheilung der Protoplasma-Partikelchen und 
Moleciilen anzunehmen haben. Und in der That, eine solche Annahme 
erscheint uns viel wahrscheinlicher, als etwa die in jiingster Zeit von 
"Goette ausgesprochene Ansicht, dass das Ei eine todte, unorganisirte 
Masse sei, oder aber als die von manchen Forschern, die sich gern mit 
