Disputed Points in Teleostean Embryology. 215 



lopment enveloped by the protoplasm of the periblast. A very 

 interesting movement of the oil-globules during development is 

 seen in that of the sole's ovum. The small oil-globules here are 

 enveloped by the cortical protoplasm, and nearly all of them 

 are at first situated in an irregular ring- of groups not far 

 from the edge of the blastoderm. As the blastoderm advances 

 the periblastic protoplasm is continually increasing in thickness 

 and extent by assimilation of the yolk, and when the ring- of 

 oil-globules is overtaken by the advancing periblast beneath 

 the blastoderm it is carried along bodily in the advance of 

 the periblast, so that ultimately the groups of oil-globules 

 are translated to a position beneath the lateral region of the 

 embryonic dorsal rudiment. Nothing could illustrate more 

 beautifully the fact that the embryonic dorsal rudiment is 

 formed by the concrescence of the two halves of the germinal 

 ring. This fact alone proves the truth of the theory of con- 

 crescence. Prof. JVPIntosh, in his review (11), says that I 

 now locate the oil-globules of the sole's ovum beneath the 

 trunk of the embryo sole. But the position in which I have 

 represented them in my l Treatise ' is the same as that in 

 which 1 represented them in my paper in the ' Journal of the 

 Marine Biological Association' in 1889. M'lntosh and 

 Prince, in their memoir (9), say that the subsequent arrange- 

 ment of the oil-globules under the developing embryo indicates 

 probably that something like a streaming of the protoplasm 

 of the periblast takes place about the period of the closure of 

 the blastopore, so as to carry the globules under the deve- 

 loping embryo. It is rare that the globules ever lie beneath 

 the axial region of the embryonic rudiment, and the supposed 

 streaming of the protoplasm is merely the coalescence of the 

 edges of the germinal ring, with its subjacent periblast, to 

 form the embryonic dorsal rudiment. The vesicular layer of 

 the yolk in the sole's egg extends pari passu with the exten- 

 sion of the periblast and blastoderm. 



Later History of the Segmentation- Gavity ', Formation 

 of the Heart , &c. 



As the blastoderm gradually increases in extent and grows 

 over the yolk the segmentation-cavity also becomes much 

 extended, and separates the epiblast from the periblast every- 

 where except beneath the embryonic shield and germinal ring. 

 It must be remembered that sections show that the mesoblast 

 layer is entirely confined to the embryonic shield and germinal 

 ring, at least until the closure of the blastopore. In eggs such as 

 those of the Salmonidge and those of Cyclopterus, Coitus, &c, 



