FORMATION OF THE GERMINAL LAYERS IN TELEOSTEI. 231 



yolk, they would naturally form a thickening on the under surface. The floor 

 of the segmentation cavity is lined by a comparatively thick layer of para- 

 blast, in which a number of free nuclei are embedded. The parablast 

 extends under the thickened peripheral portion of the blastoderm, and 

 around its margin forms a thickened welt, in which a number of nuclei are 

 also found. 



Fig. 22 represents a portion of the section more highly magnified, The 

 peripheral parablast is richly charged with nuclei, as also is that lining the 

 floor of the segmentation cavity. These nuclei, before the formation of the 

 peripheral thickening, were abundantly distributed throughout the parablast, as 

 may be seen by a reference to fig. 20, which represents a slightly earlier 

 stage. Now, however, the portion of the parablast on which the thickened rim 

 rests is very thin, and is quite devoid of nuclei, whereas both in the peripheral 

 parablast and in that portion in front of the thickening nuclei are still nume- 

 rous. It is, therefore, impossible to avoid the conclusion that the nuclei, and 

 a large portion of the protoplasm formerly situated in the region of the 

 thickened rim have been used up in the formation of that thickening. The 

 layer, which is pushed inwards from the thickening, constitutes the primitive 

 hypoblast, so that I am brought back to my former observations on the 

 development of Trachinus and Motella (6), and can only reiterate that this 

 layer is mainly if not entirely formed by a segregation of cells from the para- 

 blast. A study of fig. 22 also brings out another important point. The cells 

 of the morula rest directly on the parablast in the region of the blastodermic 

 rim, whereas the two layers are separated more centrally by the segmentation 

 cavity. The result is, that the primitive hypoblast is closely connected with 

 the epiblast of the morula in the blastodermic rim, whereas the layer as it 

 gradually fills in the segmentation cavity never adheres to the epiblast, but is 

 always distinctly separated from it by a slight remnant of the cavity itself. It 

 is in this manner that I would propose to get rid of one of the chief arguments 

 against the parablastic origin of the primitive hypoblast. It has been argued 

 by Henneguy (10) and others, that if the primitive hypoblast in Teleosteans 

 was really formed from a different source than the archiblast, the two primary 

 layers ought to be distinct throughout their entire length. In other words, that 

 the separation of the primitive hyploblast and epiblast towards the centre of the 

 embryonal shield, and their close union at the periphery, was a strong argu- 

 ment in favour of the origin of the primitive hypoblast as a true invagination 

 of the archiblast. Hoffmann holds similar views. It will, however, be easily 

 understood that this close union of the primitive layers at the periphery is 

 equally the necessary result of the views which I advocate. The segmentation 

 cavity does not extend across the whole diameter of the disc as was advocated 

 by Haeckel, but the peripheral portion of the disc always rests on the para- 



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