ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
411 
position in which the egg is placed after the operation is an important 
factor in determining whether a half embryo or a whole-embryo of half 
size will develop from one of the first two blastomeres. Mr. Morgan 
thinks that the results of both Koux and Hertwig are true, in so far as 
they apply to the embryos described by each, and the conclusions of 
each are wrong, in so far as they deny the possibility of the results of 
the other. The author thinks that the results show that the phenomena 
of half or whole development of an embryo from one of the first two 
blastomeres is entirely a protoplasmic phenomenon, the results having 
nothing whatever to do with a qualitative division of the egg, at the 
first cleavage or with a later post-generation. Whether we get a half 
or a whole embryo will depend upon the subsequent arrangement of the 
protoplasm in the uninjured blastomere, and upon the relation of the 
protoplasm to the uninjured and injured halves. If the egg is turned 
after one blastomere has been injured, so that a rotation of the contents 
of the uninjured blastomere takes place, then a whole embryo tends to 
develope. The completeness of the development depends upon the 
extent of the rotation, but this rotation of the protoplasm is not the only 
factor determining whether a half embryo or a whole embryo of half size 
is produced. 
Development of the Hypophysis.*— Prof. G. Valenti finds on the 
dorsal surface of the archenteron (in larvae of Bufo variabilis) two quite 
distinct buds. The foremost, nearest the pharyngeal membrane, is 
directed towards that ectodermic pouch which Kuppfer regards as the 
primitive mouth, and represents what Kuppfer calls the preoral intestine. 
The other, situated more dorsally, grows up between the anterior end of 
the notochord and the inferior wall of the brain, and appears to Valenti 
to give origin to the epithelial lobe of the hypophysis. According to 
Kuppfer, this epithelial lobe is due to a union of an ectodermic portion 
(primitive mouth) and an endodermic portion (preoral intestine). 
Theory of Development. f — Herr Hs. Driesch has summed up his 
views in regard to ontogeny in a book entitled ‘Analytic Theory of 
Organic Development.’ As we have previously recognised, his theory 
has grown round a series of brilliant experimental researches. 
The ovum, or its nucleus rather, is a mixture of potentialities, but 
there is no structural preformation beyond what is implied, and that is 
not a little, in the “ form.” These potentialities are expressed in the 
successive steps of development, which are induced by stimuli and de- 
termined by the mutual relations of parts. The formative factors are 
either chemical or physical ; thus an organism may grow chemically and 
actively by assimilation, or physically and passively by absorption of 
water. Changes are induced partly by internal factors, such as the 
influence exerted by the nucleus ( a ) in virtue of its position, ( b ) by its 
“ induction.” External factors, such as gravitation, are also operative. 
The internal physical “ inductions ” are conditions of tension, pressure, 
contact, and mass ; others are chemical. As to the succession of events, 
* Anat. Anzeig., x. (1895) pp. 538-40 (1 fig.). 
f ‘ Analytische Theorie der organischen Entwicklung,’ Leipzig, 1894, 8vo, xiv. 
and 185 pp., 8 figs. ; reviewed by Dr. T. Garbowski, Biol. Centralbl., xv. (1895) 
pp. 306-32. 
