114 DR. E. KLEIN. 



by WeiP^ and Romiti.^^ The appearance of the segmentation 

 cavity, the difi'erentiation of the blastoderm, while growing 

 round the yolk-sphere, into a thinner central and a thicker 

 peripheral part, the foundation of the embryo, and the de- 

 velopment of the embyonal layers, were described in accord- 

 ance with Strieker and Rieneck.^ The development of the 

 central nervous system was mentioned in conformity with 

 the assertions of Kupfer* on this subject. 



Oellacher, in his paper^^ on the development of the embryo 

 and the embryonal layers, has made some very minute and 

 detailed observations, which to some extent tend to modify 

 the previous accounts of these matters. 



Thus, Oellacher shows that the segmentation or subgerminal 

 cavity does not appear, at the outset, between the central part 

 of the blastoderm and the yolk, but is founded excentrically, 

 in consequence, as it were, of the peripheral thickening of 

 the blastoderm being much greater and broader on one side 

 than on the other. This statement I can fully confirm (see 

 fig. 4). As this, however, does not form the chief subject of 

 my present inquiry, I will not pursue it further. 



Comparing thin sections of the blastoderm of different 

 stages of early development which have been hardened in 

 dilute chromic acid (one sixth per cent.), and then prepared 

 in the ordinary way, I find that besides the blastoderm proper 

 in its different aspects during early segmentation (compare 

 figs. 1 — 4), the subgerminal and paragerminal substance 

 deserves equally close attention, for this substance bears at all 

 stages of development an important genetic relation to the 

 blastoderm and the embryo. 



As is well known, the blastoderm of the trout and of Teleos- 

 tean fishes in general lies closely upon the surface of the yolk- 

 sphere in a slight depression of the latter, round which 

 most of the oil or fat globules accumulate immediately after 

 fertilization. As segmentation proceeds, the blastoderm 

 becomes gradually separated from the surface of the yolk- 

 sphere by the appearance of a cleft, i. e. the subgerminal 

 or segmentation cavity. From the earliest stages of segmenta- 

 tion — say, from a period when the blastoderm appears on 

 section to be a finely granular mass, which, by the presence 

 of a few more or less distinct perpendicular and horizontal 

 fissures is divided into a certain limited number of contiguous 



* The Eeporter in the ' Jahresbericht,' while mentioning this observation 

 of Weil, states, anticipando, that Romiti, whose researches were conducted 

 in the Reporter's laboratory, arrived at similar results ; while writing so the 

 Reporter is altogether unaware of my own description of the same pheno- 

 mena. 



