254 Analytical Notices of Books. 
Von Baer’s observations on the ova of Mammalia, Hensand Frogs, 
and the authour’s own researches on those of Blennius viviparus, fur- 
nish one side of his comparative view of the structure and developement 
of Vertebrata and the Crawfish. The first remarkable difference between 
them consists in the diffusion of the embryo over the whole surface of 
the vitellus in the latter, previously to its contraction towards a determinate 
centre; an appearance which has never been observed in the former. 
‘The difference in the form of that body, when it first becomes visible, 
assuming the shape of a carina (so called) in Vertebrata, and that of a 
half ellipse in the Crawfish, appears to be of less importance. The 
anatomical structure of Vertebrata consists primarily of an external or 
serous membrane, an internal or mucous, and a vascular tissue inter- 
posed between them. In the Crawfish the latter appears to be wanting, 
and the vascular parts seem to be immediately derived from the serous 
membrane. Generally speaking, however, the same organs are in both 
cases produced by the same membrane. These observations apply equally 
to the ova of Spiders. On this point Dr. Rathke observes that the want 
of a proper vascular tissue in the embryo of Annulosa is in all probabi- 
lity the reason why these animals have no such parenchymatous intestines 
as the Vertebrata, all their secretory and excretory organs appearing 
only as discrete tubes without parenchymatous envelopes. 
Of the two membranes, the most important in the formation of the 
embryo is the serous, which is developed in a very different manner in 
the Crawfish, and in Vertebrata. We cannot here follow the authour in 
his minute details, but must content ourselves with stating that he adopts 
Von Baer’s type of the embryo in Vertebrata, as consisting of a double 
convolution of the embryonal sacculus proceeding upwards and down- 
wards from a middle line; and opposes to it the type of the embryo of 
the Crawfish, and probably of all Annulosa, as formed of a simple con- 
inferior station in the natural arrangement of Crustacea. At a more advanced 
period the two series of ganglions in the foetal Crawfish approach the medial 
line on either side, become united together, and form a single chain, which 
corresponds exactly with the structure of the same organ in the adult Cymothoe. 
And lastly the whole series of ganglions run together longitudinally, so as to 
form in the adult Crawfish a simple nervous cord, like that of the more highly 
developed animals of the Class. Such comparisons open an ample field of phi- 
losophical consideration, 
