FORMATION OF THE GERMINAL LAYERS IN TELEOSTEI. 233 



outline, and seem to fuse together to form a more solid mass. This is the 

 commencement of the notochord. The two lateral portions become separated 

 from it as the lateral mesoblastic plates, and a single row of cells remains in 

 connection with the parablast, which constitute the commencement of the 

 permanent hypoblast. The lateral plates of mesoblast are completely separated 

 from the epiblast, but above the notochord there is an arrangement of the cells 

 which would lead one to suppose the two had been in close union. It thus 

 appears that in the herring the three germinal layers are not completely 

 differentiated until the notochord has made its appearance to separate the two 

 lateral plates of mesoblast. Though I cannot speak with absolute certainty, 

 there appears every probability that the primitive hypoblast gives rise to 

 notochord, mesoblast, and permanent hypoblast, and that the morula mass of 

 cells existing prior to the formation of the primitive hypoblast (the archiblast 

 in other Teleostean types) persists as the epiblast. It is possible, however, but 

 by no means sure, that some of its cells are included in the upper rows of 

 mesoblast cells. 



Theoretical Considerations. — Since my observations were completed and the 

 bulk of the present paper written, I have received a paper by Dr Ruckert (25) 

 on the Formation of the Germinal Layers in Elasmobranchs, published about 

 six months ago, in which the author has come to very similar conclusions to 

 those here advocated. His observations were made chiefly on the eggs of 

 Torpedo, and the following is a short summary of the results arrived at : — 



1. The free nuclei in the yolk of meroblastic eggs (which Ruckert terms 



merocytes) are segmentation products which have undergone a secondary 

 modification under the influence of the food yolk. Their mode of origin 

 is most clearly demonstrated by the peculiar segmentation of the richly 

 deutoplasmic but still holoblastic eggs of many invertebrates (Insecta, 

 Crustacea, Vermes). In these there is at first a total segmentation, but 

 later the nuclei in the vegetative pole, together with the surrounding- 

 protoplasm, separate themselves from the deutoplasm, and while under- 

 going frequent division produce embryonal cells which are undistinguish- 

 able from those formed by regular segmentation. The deutoplasm in 

 the vegetative pole becomes by this means passive food material, and the 

 whole original holoblastic egg a meroblastic one. 



2. In Elasmobranchs the merocytes are amoeboid (rhizopodenartig) structures 



whose richly ramifying processes absorb and assimilate the surrounding 

 yolk. They produce later a number of embryonal cells by endogenous 

 cell formation or budding. 



3. The embryonal cells produced from merocytes take part in the formation 



of all the germinal layers. In most animals they form the entoblast, 



