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



enclodermic cells. When at its maximum it is a slightly flattened dome-like cavity 

 (PI. II. fig. 156, gc) ; but with the extension of the blastoderm its roof is depressed, 

 and it thus appears subsequently as a mere fissure. Now Lereboullet figures his 

 cavity as a narrow fissure extending almost from margin to margin of the blastoderm ; 

 whereas Bambeke's is a compact, but loftier and more spacious chamber.* It is 

 noteworthy that Bambeke was struck by this dissimilarity, and after examining the 

 segmentation-cavity in the roach was prompted to seek for a germinal cavity underneath 

 the blastoderm, and found one, as he indicates in his figs. 4 and 6 (vide No. 20a) ; 

 but he adds that " a comparative examination of preparations forces me to regard it as a 

 simple accident and artificial, for the prominences and depressions of roof and floor 

 coincide." There is much reason to suppose, therefore, from the shape and nature of 

 the floor, that Lereboullet's cavity is not a segmentation-cavity, such as Bambeke 

 supposes, and, if this be so, then Lereboullet likewise discovered this flattened 

 germinal cavity, as E. van Beneoen says (No. 25, p. 47), though this author is wrong 

 in according the discovery also to Van Bambeke. If, on the other hand, Lereboullet's 

 be really Von Baer's (and Van Bambeke's) cavity, then H. Rathke first signalised the 

 germinal cavity in Zoarces ; and he was followed by Stricker. It is therefore not correct 

 to speak of a cavity of Lereboullet with Van Beneden but rather of a (sub-blastodermic) 

 germinal cavity, which is persistent through all embryonic life, as distinct from the (intra- 

 blastodermic) segmentation-cavity which wholly disappears.t 



What then is the significance of the germinal cavity thus distinguished ? By the 

 fact that its floor is formed of yolk, or rather the protoplasmic cortical film (or inter- 

 mediary layer), and that it is roofed over by endoderm (lower layer) and epiblast-cells, it 

 is comparable to the " Keimhohle " in the fowl's ovum.| At a later stage the hypoblast- 

 cells which intrude from the periphery to form the blastodermic rim (br) and shield 

 (PL II. figs. 15, Or- e, and 17) do not pass across the floor of the cavity, but creep 

 up the sides and partially arch it over, forming in fact a gastrula which would open ex- 

 ternally by the blastopore, were not this aperture plugged up by the mass of yolk (really 

 Ecker's plug), which is so large that the invaginated lip is compelled to pass round, and 

 epibolically envelop it. The germinal cavity, arched over as it is by the thick blastodermic 

 roof, bdm (PI. II. fig. 15, a-e), is never truly open in the sense indicated; but potentially it 

 is so, the removal of the concentrated trophic matter (y) which does not segment would leave 

 the blastoderm a simple gastrula — indeed, as Ryder remarks in regard to Alosa, that 

 " the yelk might be removed at any stage without taking away any essential part of the 

 embryo except the floor of the cavity " (No. 141, p. 569). Van Bambeke does not hesitate 

 to regard his chamber as a gastrula-cavity, and finds in it therefore great phylogenetic 



* A glance at Lereboullet's figure (No. 93, pi. iii. fig. 3) and Bambeke's (No. 20a) sufficiently shows this. 



t See a paper " On the Significance of the Yolk in the Eggs of Osseous Fishes," by E. E. Prince, Ann. Nat. Hist., 

 July 1887. 



X It is interesting to observe that, with the appearance of the germinal cavity, the thick periblast-floor in some forms 

 becomes thinner. The Keimhohle or germinal cavity is often called the segmentation-cavity in the fowl's ovum. 



