336 F. M, BALFOUR. 



is first formed, it is only on each side of the middle line that 

 the difFeventiation of the lower layer cells into hypoblast and 

 mesoblast takes place, I do not certainly know, but it is 

 undoubtedly a very early condition of the mesoblast. The 

 condition of the mesoblast as two plates, one on each side of 

 the neural canal, is precisely similar to its embryonic condi- 

 tion in many of the Vermes, e. g. Euaxes and Lumbricus, In 

 these there are two plates of mesoblast, one on each side of 

 the nervous cord, which are knoAvn as the Germinal streaks 

 (Keimstreifen) [vide Kowalevsky Wiirmern u. Arthropoden; 

 Mem. de I'Acad. Imp. St. Petersbourg, 1871). 



From longitudinal sections I have found that the seg- 

 mentation cavity has ceased by this stage to have any distinct 

 existence, but that the whole space between the epiblast 

 and the yolk is filled up with a mass of elongated cells, which 

 probably are solely concerned in the formation of the vas- 

 cular system. The thickened posterior edge of the blastoderm 

 is still visible. 



At the embryonic end of the blastoderm, as I pointed out 

 in an earlier stage, the epiblast and the lower layer cells are 

 j)erfectly continuous. 



Where they join the epiblast, the lower layer cells become 

 distinctly divided, and this division commenced even in the 

 earlier stage, into two layers ; a lower one, more directly con- 

 tinuous with the epiblast, consisting of cells somewhat re- 

 sembling the epiblast-cells, and an upper one of more 

 flattened cells (PI. XIII, fig. 4, m). The first of these forms 

 the hypoblast, and the latter the mesoblast. They are indi- 

 cated by hy and m in the figures. The hypoblast, as I said 

 before, remains continuous with the whole of the rest of 

 lower layer cells of the blastoderm {vide fig. 7 b). This 

 division into hypoblast and mesoblast commences at the 

 earlier stage, but becomes much more marked during this 

 one. 



In describing the formation of the hypoblast and mesoblast 

 in this way I have assumed that they are formed out of the 

 large mass of lower layer cells which underlie the epiblast at 

 the embryonic end of the blastoderm. But there is another 

 and, in some ways, rather a tempting view, viz. to suppose 

 that the epiblast, where it becomes continuous with the hy- 

 poblast, in reality becomes involuted, and that from this 

 involuted epiblast are formed the whole mesoblast and 

 hypoblast. 



In this case we would be compelled to suppose that the 

 mass of lower layer cells which forms the embryonic swelling 

 is used as food for the growth of the involuted epiblast, or 



