196 Scientific Intelligence. — Zoology. 



servers in the ova of other vertebrata, mentions some essential differ- 

 ences between his own observations and those of others as to the nature 

 and mode of origin of these objects, and their relation to surrounding 

 parts. Von Bacr, the discoverer of the chorda dorsalis^ describes this 

 structure as " the axis around which the first parts of the foetus form/ 

 Reichert supposes it to be that embryonic structure which serves as 

 " a support and stay" for parts developed in two halves. The author's 

 observations induce him to believe that, instead of being " the axis 

 around vrhich the first parts of the fcetus form/' the incipient chorda is 

 the last- formed row of cells, w^hicli have pushed previously-formed cells 

 farther out, and that, instead of being merely ^' a support and stay" 

 for parts developed in two halves, the incipient chorda occupies the 

 centre out of which the " two halves " originally proceeded as a single 

 structure, and is itself in the course of being enlarged by the continued 

 origin of fresh substance in its most internal part. 



The author enters into a minute comparison of the objects in ques- 

 tion ; from which it appears that the incipient chorda is not, as Baer 

 supposed, developed into a globular form at the fore end, but that the 

 linear part is a process from the globular ; and that the pellucid cavity 

 contained within the latter — a part of prime importance, being the 

 main centre for the origin of new substance — is not mentioned by Von 

 Baer. Farther, that the origin of the " lamina; dorsales" of this natu- 

 ralist (the " central nervous system" of Reichert) is not simultaneous 

 with, but anterior to, that of the chorda. 



The author then reviews the observations of Rathke and Reichert 

 on the chorda dorsalis, which contain internal evidence, he thinks, of a 

 process in the development of fishes, reptiles, and birds, the same as 

 that which he has observed in mammalia ; namely, the origin of the 

 embryo out of the nucleus of a cell. 



And it is his opinion that this observation may assist to solve a 

 question on which physiologists are not agreed ; for it shews that if 

 the nucleus of a cell is a single object, the first rudiments of the embryo 

 are not two halves. The author thinks that, unless the very earliest 

 periods are investigated, it is in vain that we attempt to learn what 

 that is of which the rudiments of the embryo are composed. From 

 not attending to this, physiologists have supposed their '' primitive 

 trace" to arise in the substance of a membrane, which the author, in 

 his second series on the embryo, shewed could not be the case. To the 

 same cause he thinks is referable an opinion recently advanced by 

 Reichert, that the first traces of the new being are derived from edh 

 ^»f the yelk* 



