224 MR GEORGE BROOK ON THE 



Leuciscus, under the name of the " couch e interme'diaire." Klein and Van 

 Bambeke differ considerably in their accounts of the early shape and position 

 of this layer, but it seems probable that both authors are correct so far as the 

 species studied is concerned, and that the parablast varies in position in 

 different species and at different stages of development. Klein thus describes 

 the appearance of nuclei in the parablast : — " Searching carefully through 

 the parablast with a moderately high power (Hartnack's No. 8) we detect 

 numerous isolated, small, transparent bodies, very faintly outlined, so as to 

 be rendered just perceptible; between these and distinct nuclei all inter- 

 mediate forms may be met with as regards general aspect, outline, and size. This 

 obviously means new formation of nuclei. It therefore stands to reason to 

 assume that, inasmuch as at a period when nuclei may be seen to multiply by 

 division, the formation of nuclei de novo, as it were, still takes place in the 

 parablast, the first nuclei of the 'parablast have also originated in the same 

 manner, i.e., de novo." 



Klein is of opinion that the peripheral thickening of the archiblast is 

 caused by an addition of cells from the subjacent parablast, and that a large 

 part of the hypoblast is derived from this layer. The evidence of Van 

 Bambeke also points to a similar conclusion. 



Kingsley and Conn (15) describe an " intermediary layer " in the pelagic 

 ova of several fishes, in which " free cell formation " takes place, but are 

 inclined to regard the true hypoblast as derived, in the first instance, from 

 an invagination of the ectodermal layer of the epiblast. Mr Kingsley, how- 

 ever, informs me that, since the publication of the paper referred to, he has 

 been led to change his views on the subject. 



Hoffmann (14) affirms that the parablast arises with the formation of the 

 first furrow, which takes an equatorial direction, thus dividing the germinal disc 

 into two layers, each of which contains half of the first segmentation nucleus. 

 According to this author, nuclei are abundant in the parablast during later 

 stages, but the layer is not destined to take part in the formation of the 

 embryo. The parablast, according to Hoffmann's view, is rather to be 

 regarded as a degenerate relic of the vegetative pole in holoblastic types, 

 which, owing to the increase of yolk, is no longer able to fulfil its former 

 functions. 



According to Agassiz and Whitman (1), the nuclei found in the parablast 

 (periblast) are derived from the margin of segmented blastoderm (archiblast), 

 and these authors figure a sixteen-cell stage, showing the origin of the 

 nucleated subgerminal layer. It would appear, however, from their observa- 

 tions, that the parablast is not concerned in formation of the hypoblast, but 

 that this is formed by a process of true invagination. 



Cunningham (8) supports the views of Agassiz and Whitman regarding 



