RECENT DISCOVERIES IN INSECT EMBRYOGENY. 
129 
A general doctrine of ovology is propounded by Dr. Meinert 
in his second memoir, which is briefly this : — In mammalia, and 
most of the invertebrata, the ovum consists of a simple germinal 
cell, of which the nucleus is the so-called germinal vesicle. 
This germinal cell undergoes segmentation, and is thus multi- 
plied by subdivision into minute embryonal cells. A portion of 
those not required for the formative development of the embryo 
is reserved for the reproductive organs of the new individual, 
which contain the respective sperm and germ elements, to be 
afterwards developed in future generations. In birds, reptiles, 
fishes, &c., the ovum receives, in addition to the germinal cell, 
either a number of yolk cells, or a secretion from them constitutes 
a yolk. These vitelline cells do not, like the germinal cell, un- 
dergo morphologic development, but pass, without any forma- 
tive changes, to the nutritive yolh. In almost all insects there 
is, besides, the germinal cell, a vitelline secretion. The resi- 
dual cells derived from endogenous multiplication of the germi- 
nal cell are, in the insect, the essential part of the tissue, which, 
by transformation, becomes the fatty substance. Whatever value 
may be attached to this scheme of ovology, the physiological dif- 
ficulties of genesis remain still unsolved, and the doctrine of 
Parthenogenesis remains, as before, a riddle. When Meinert 
adds to his anatomical scheme the assumption that the influence 
of a male element is not indispensable — not even necessary— to 
the development of the germ cell in all those invertebrata cha- 
racterised by non-sexual propagation, the ^conclusion may be con- 
sidered premature, to say the least. The exceptions to a law 
almost universal are neither so absolutely confirmed, nor so 
numerous, as to disprove its absolute character. The dimor- 
phism of all creatures (male and female), which seems to be inse- 
parably connected with the law of genesis, is preserved equally 
in the creatures temporarily multiplying by immature genera- 
tions. And again, the combination of male and female elements 
for the production of a new creature, may not be at once ob- 
vious, because the types of reproductive organisation may really 
vary more than we are at present aware of. For instance, forms 
of hermaphroditism, still more rudimentary than yet observed, 
might easily be imagined. We are not, therefore, warranted in 
rejecting the widest application of the known genetic law, be- 
cause cases occur in which it is masked by the occurrence of 
special intermediate phases of life, the further study of which 
may completely confirm and rehabilitate the single and absolute 
law of genesis. 
