ii54 Analytical Notices of Books. 



Von Baer's observations on the ova of Mammalia, Hens and 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 Vtrtehrata and the Crawfish. The first remarkable difference between 

 them consists in the diifusion of the embryo over the whole surface of 

 the vltellus 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 Fertebrata, and that of a 

 half ellipse in the Crawfish, appears to be of less importance. The 

 anatomical structure of Vertehrata 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 Jlnnulosa is in all probabi- 

 lity the reason why these animals have no such parenchymatous intestines 

 as the Vertehrata, 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 Vertehrata. 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 Vertehrata, 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 fcEtal 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. 



