Disputed Points in Teleostean Embryology. 205 



number of yolk-spherules of various sizes, strictly comparable 

 with the yolk-spherules of an Elasmobranch or Sauropsidan. 

 In all cases where such separate yolk-spherules occur they are 

 separated by strands and bands of protoplasm which are ulti- 

 mately continuous with the protoplasm of the blastodisc and 

 the cortical protoplasm of the vitellus. 



I pointed out in my paper in the l Journal of the Marine 

 Biological Association ' (4) that the continuous homogeneous 

 yolk of the typical pelagic ovum was to be regarded as a single 

 large yolk-sphere representing the numerous yolk-spherules 

 of such an egg as that of the herring or salmon, fused together, 

 all the protoplasm being thus driven to the external surface of 

 the vitellus. And I also pointed out that eggs like that of 

 the sole with a single external layer of yolk-segments formed 

 an intermediate condition, in which, while most of the yolk- 

 spherules had fused together, a few still remained separated 

 by protoplasmic partitions. Development shows this to be 

 the true account of the matter, for in the ovarian development 

 of typical pelagic ova the homogeneous yolk is actually pro- 

 duced by the coalescence of distinct yolk-spherules. Agassiz 

 and Whitman, in one of their beautiful memoirs (7), describe 

 a layer of yolk-segments exactly similar to that in the sole, in 

 an egg which they identify as that of Temnodon saltator. 

 They fail to find any explanation of these yolk-segments, 

 but they distinctly retract their previous conclusion that 

 the segments indicated a partial segmentation of the yolk 

 connected with the segmentation of the blastoderm, and 

 partially representing the segmentation of the yolk in the 

 Amphibian ovum. Examination of sections has shown them 

 that these yolk-segments have nothing to do with either the 

 segmentation of the blastoderm or the nuclei of the periblast. 



In spite of this retraction by Agassiz and Whitman of 

 their previous conclusion M'Intosh and Prince in their large 

 memoir (9, p. 720) still refer to the yolk-segments in the 

 eggs of Temnodon saltator and of the common sole as a rudi- 

 mentary survival of the nucleated yolk-cells formed by the 

 process of segmentation in the ova of Amphibians, Petro- 

 myzon, &c. The memoir of Agassiz and Whitman in which 

 they repudiate this interpretation is mentioned by M'Intosh 

 and Prince in their bibliographical list ; but apparently Pro- 

 fessor Prince, who we are told is responsible for the embryo- 

 logical part of the memoir, had not read all the works he refers 

 to. Professor Prince does not allude to the obvious obstacle 

 to any comparison between the yolk-segments in the egg of 

 the sole and the yolk-cells of the Amphibian ovum, namely 

 that the former are developed in the ovary long before fertili- 



