710 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



structures is very probable ; but the recognition of a cavity in the Teleostean blastoderm 

 has been so general that it cannot be placed in such a category. 



We speak of it as a " germinal cavity," and do so advisedly, for it is not " the cavity 

 of Von Baer," better known as the blastoccel or segmentation-cavity. This latter, which 

 exists in all segmented germs forming a blastosphere, as in Cylostomes and Amphioxus, 

 is, we believe, never formed in such pelagic ova as are referred to here, nor indeed has it 

 been clearly recognised in any other Teleostean ova, with the exception of Leuciscus 

 rutilus. In this last named ovum Van Bambeke fully describes a true " segmentation 

 cavity," though his results are not in accordance with those of embryologists generally. 

 Van Bambeke himself doubts the existence of his cavity in the germ of the Salmonoids 

 and carps, though closely allied to the form he investigated, and declares it to be 

 homologous with the chamber in the ovum of Petromyzon, Acipenser, the Selachians, and 

 Amphibians. It is true he quotes Lereboullet in support of his view, and the latter 

 undoubtedly does speak of the germ at the close of segmentation as having " un aspect 

 granuleux et la forme d'une sphere aplatie qui repose sur le vitellus " (No. 93, p. 503) ; 

 but neither his fig. 27, pi. i. nor fig. 3, pi. iii. necessarily imply Bambeke's results, 

 nor exclude the existence of the germinal cavity which most authors have seen. The 

 segmentation-cavity of Bambeke, the homologue of Balfour's cavity (No. 13, pi. xxi. 

 fig. 1, sc), arises as a space in the midst of the blastodermic mass (No. 20a), at 

 what period he cannot say, though his figure would indicate an early stage, 

 probably when the blastoderm covers a quadrant, that is at the same time as the 

 " germinal " cavity, which it also resembles in its non-central position, for it is slightly 

 eccentric in position, and in front of the embryonic area proper. It is surrounded by 

 blastomeres — the roof, walls, and floor being composed of cells produced by the 

 segmentation of the disc. The germ, in which it originates, is essentially a blasto- 

 sphere, for though the floor-cells largely disappear, so that the yolk may seem 

 partially to form the floor, there is probably never a stage, as Balfour is careful to note 

 (No. 11, p. 519), " in which the floor of the cavity is without cells." Balfour, it is true, 

 regards the Teleostean germinal cavity as homologous with the segmentation-cavity 

 (cavity of Von Baer) in Elasmobranchs and Amphibians (No. 10, i. p. 70) ; but the 

 subsequent fate of each of these cavities tells against this homology, for the former is 

 persistent, whereas the latter chamber is transitory. If the Teleostean germ after 

 segmentation be a morula, which flattens out, and becomes lifted up, and separated by a 

 chamber from the appended trophic mass,* resembling in a remarkable manner the 

 condition in certain Urochordates (e.g., the cauducichordate Pyrosoma), in which no 

 centrally placed segmentation-cavity occurs (vide Huxley, No. 73, and Kowalewsky, 

 No. 86, p. 609), then the presence of such a cavity, and the occurrence of a blastospherical 

 stage in Teleosteans, must be regarded as problematical. 



* That the blastoderm is actually raised up seems to be demonstrated by the fact that separation may for some time 

 be incomplete, connecting strands of protoplasm being frequently distinguishable in the living ovum and in sections 

 (PI. II. fig. 15, c), and Ryder is probably in error when he supposes the cavity to arise as a direct result of cleavage 

 (No. 141, p. 492). 



