ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
15 
performance of its work by tbe cytoplast ; (d) to the imperfect nutrition 
of the nucleus ; and (e) to the failure of tbe cell as an organic whole. 
Replacement theories of fertilization are inadmissible, since all fail 
to account for one or more of the following facts : — 
i. Multiple isogamy. 
ii. The non-discrimination of the broods of exo-isogamotes into two 
categories, of which the members of either would pair with 
those of the other category, but not of their own. 
iii. The absence of “ excretion phenomena ” of any kind in so many 
cases of gametogeny. 
iv. The existence of true parthenogenesis of male as well as female 
gametes. 
v. The formation of a male individual from the exclusively female 
oosphere of the Hive-bee. 
Elements of Development of Domestic Mammalia.* — Prof. R. 
Bonnet has published a textbook of the embryology of domestic Mammals 
which is reviewed by Dr. F. Maurer. The critic points out that this is 
the first attempt to treat of the development of the Mammalia in a 
thorough manner and with the use of the latest literature. In most 
textbooks of embryology Man is more or less made the centre of the 
description, and Mammals, other Vertebrates, and Invertebrates are used 
to fill up lacunm. Our knowledge of the embryology of Ungulates is 
especially increased. 
The first part of the book deals with the development of the body- 
form ; the formation of the two primary germinal layers is explained as 
the division into two plates, one lying above the other, of the remains of 
the cleavage cell that forms the embryonic spot ; the primary ectoblast 
(Rauber’s layer) is not considered to have the value of a germinal layer, 
and it disappears later on. The median layer is regarded as a secondary 
product of the tw r o primary layers ; the author divides it into two parts 
— the epithelial primitive stripe with its cephalic process and an inter- 
mediate tissue which is called mesenchym : this grows between the two 
primary layers and functions as a supporting and packing tissue. By 
mesenchym Bonnet means something quite else than the Hertwigs, and 
the views of the latter are shown to be untenable. 
The second and largest part of the book treats of tbe development 
of the organs and systems. No observations are found to support the 
doctrine that the nerve-fibres grow out from the neuroblasts. In the 
third part of the book the ovarian envelopes are described, and there is 
an account of the embryonic circulation and of the phenomena of birth. 
Development of Seps.j — Dr. E. Giacomini has studied the develop- 
ment of Seps chnlcides. This lizard is viviparous; the development of 
the ova takes about three months ; the number of eggs which descend 
into the oviducts varies from 5 to 15. The ovarian ova do not exceed 
2-5-3 mm. in diameter, and have not much yolk. After fertilization 
each lies in an incubatory chamber in the oviduct, without secondary 
membranes or albumen ; they increase in size till they measure 21-22 
* ‘ Grundriss der Ent.wiekelungsgescliichte der Haussaugethiere,’ von Prof. R. 
Bonnet, Berlin, 1801, 282 pp. and 201 figs. See Morphol. Jalirbuch, xvii. (ISO!) 
pp. 681-3. f Monitore Zool. Ital., ii. (1891) pp. 176-92, 198-209 (1 pi ). 
