120 DR. E. KLEIN, 



to say, may correspond to the layer of cells observed by 

 Kupffer (see above) around the blastoderm of Gasterosteus 

 and Spinachia. As we have mentioned before, Kupfer 

 makes these cells originate after the mode of endogenous 

 cell-formation. 



Balfour,^* in speaking of the later stages of segmentation of 

 the ovum of Elasmobranch fishes, says (1. c, p. 4) : " The 

 limits of the blastoderm are not defined by the already com- 

 pleted segments, but outside these new segments continue to 

 be formed around nuclei which appear in the yolk. At this 

 stage there is, therefore, no line of demarcation between the 

 germ and the yolk ; but the yolk is being bored into, so to 

 speak, by a continuous process of fresh segmentation." 

 Balfour then goes on : " Intimately connected with the seg- 

 mentation is the appearance and history of a number of 

 nuclei which arise in the yolk surrounding the blastoderm. 



The blastoderm thus rests upon a mass of finely 



granular material, from which, however, it is sharply sepa- 

 rated. At this time there appear in this finely granular 

 material a number of nuclei of a rather peculiar character." 

 These nuclei vary, according to Balfour, very much in size. 

 *' They are rather irregular in shape, with a tendency when 

 small to be roundish, and are divided by a number of lines 

 into distinct areas, in each of which a nucleolus is to be seen. 

 The lines dividing them have a tendency to radiate from the 

 centre." These very peculiar nuclei are scattered through the 

 subgerminal granular matter; they are distributed in a special 

 manner under the floor of the segmentation cavity, on which 

 new cells are continually appearing, and are identical with 

 those present in the cells of the blastoderm. From these 

 facts Balfour concludes that the " nuclei of the yolk" become 

 actually the nuclei of cells which enter the blastoderm ; they 

 are later on concerned partly in the formation of the vascular 

 system, and still more in that of " the walls of the digestive 

 canal and of other parts." 



Haeckel,^' in his studies of the development of Teleostean 

 fishes (probably Lota), finds that only the cells resulting from 

 the segmentation of the blastoderm participate in the forma- 

 tion of the developing body of the fish, all the rest of the 

 ovum, i. e. food yolk, is homogeneous and amorphous, and has 

 nothing to do with the development of the blastoderm. 

 ''Thus,'' Haeckel goes on to say (1. c, p. 96), "the false 

 ' Parablast theory of His,' and all similar theories, according 

 to which in discoblastic vertebrate ova histogenetic embryo- 

 cells are said to develop out of the separate food yolk, inde- 

 pendently of the primary embryonal layers, are hereby ' biindig 



