492 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [38] 



111 18G5 Strieker discovered tbis cavity in the egg of the trout, in 

 consequence of which (EHacher has proposed to name it after him. 

 This cavity is altogether different from what has been described bj 

 several authors as appearing in the center of the celhilar mass of the 

 disk, and which, as suggested by OEllacher, is, in all probability, a prod- 

 uct of reagents. CEllacher also appears to have been aware of the 

 persistence of this cavity up to the time when the yelk blastopore closes. 

 So that so far as 1 may have any claims to priority in the matter it simply 

 relates to a proof that it persists in a considerable number of genera 

 and is characteristic of the blastoderm oi Teleosts in general. OEllacher, 

 however, does not regard it as a segmentation cavity, so that it has re- 

 mained for Balfour and myself to establish its homology. I at one time 

 believed with Balfour that this cavity was at first closed below by a 

 more or less complete stratum of cells, but if its development is followed 

 to the time when the yelk blastopore is closed it will be found that such 

 is not the case, and that tlie yelk hypoblast, which is not truly hypo- 

 blastic, corresponds simply to the granular layer of Balfour. When I 

 say that the yelk envelope, DotterJiaut, couche intermSdiaire^ yelk hypo- 

 blast, etc., as it has been variously named, is not truly hypoblastic, I 

 mean to imply that no portion of it is in the relation of a hypoblast to 

 the embryo and that it seems to serve simply to inclose the yelk and ef- 

 fect its metamorphosis into blood. The few scattered nuclei in its sub- 

 stance, except just below the embrj^o's head, cannot be regarded as 

 forming a cellular tloor, since in preparations stained with borax carmine 

 these nuclei are seen to be free, much scattered, and simply involved 

 in the plasma of this layer. 



It will be seen from the above that my views have undergone some 

 change since the publication of my paper on Tylosurm ; but these 

 changes of opinion relate entirely to the history and fate of the yelk 

 envelope or intermediary layer of Van Bambeke. I still hold to my 

 interpretation of the homology of the whole amphibian ovum with the 

 disk only of the Teleosteau egg ; but a discussion of this and other the- 

 oretical matters may be more fittingly reserved for the close of this 

 l)aper. 



In other types the segmentation cavity is unquestionably originated 

 as a direct result of cleavage. This is apparently the case in the Tel- 

 eostean egg ; after the blastoderm is fairly formed the germinal mass 

 of cells immediately involved in the development of the embryo, having 

 been detached during segmentation from the germinal matter compris- 

 ing the yelk envelope, they are freed in great measure from complete 

 contact with the latter except underneath the embryonic disk. Fluid 

 finds access into the cavity beneath the thinner non-embryonic portion 

 of the blastoderm, but as the blastoderm grows the cavity increases in 

 dimensions transversely and diminishes in depth, so that finally a film 

 of fluid of extreme tenuity is interposed between the non-embryonic 

 portion of the blastoderm and the homogeneous yelk membrane. Viewed 



