672 THE RESPIRATION AND [pt. iii 



assumptions, the nature of which will presently become clear. In 

 order to understand his point of view, attention should again be 

 directed to Fig. 124, in which his data are presented for the carbon 

 dioxide given off by the embryos of the minnow and the frog during 

 their development. Both show the preliminary rise on segmentation, 

 but the frog curve descends sooner, and then rises steadily to reach 

 a peak about 60 hours after fertilisation, at the same time as the 

 minnow. From that point onwards, the amphibian and the teleost 

 run closely together. The development of the frog, as Bataillon 

 pointed out, is, in fact, more rapid in the early stages. The occlusion 

 of the yolk-plug occurs earlier relatively in the frog than in the 

 minnow, yet the subsequent work of embryonic organisation takes 

 a longer time in the former than in the latter. In the frog, between 

 the initial segmentations and the period of spread of the ectoderm over 

 the yolk there is, as can be seen, a short trough, after which the respira- 

 tory activity mounts steadily till the time comes for the closure of the 

 yolk-plug. In the teleost, on the contrary, the period of extension 

 of the blastoderm is short, and is preceded by a well-marked stasis, 

 as if the development was meeting a persistent and not easily con- 

 quered obstacle. This once overcome, the extension of the blastoderm 

 over the yolk goes on rapidly, accompanied by the rapid upstroke 

 of the respiration curve. The embryo of the minnow having arrived 

 at the 1 8th hour is constantly enriching itself with new cells, especially 

 on its under surface. The periblastic elements are exercising a kind 

 of sorting action on the yolk, by which it furnishes an abundance 

 of chromatin material for future cell-divisions. But towards the 30th 

 hour the spider-Uke forms of cells characteristic of the earlier stage 

 have disappeared, and there is a very sharp boundary-line between 

 the yolk and the periblast, all the cells of which appear to be at 

 rest, very few mitotic figures being visible. In the embryo itself 

 exactly the same state of affairs has come about ; the cell-divisions 

 are very rare even on the under surface, where they are mostly to 

 be found at the edges. There is, in fact, a period of temporary cessa- 

 tion of mitotic activity, and of slowing down of developmental rate. 

 Bataillon thought that the reason for this was the heaping up of the 

 layers of cells, as many as fifteen being demonstrable in certain 

 regions, so that the inner ones were cut off from the air, and the outer 

 ones from the nutritive materials of the yolk. In such circumstances 

 the metazoal embryo might be considered to have gone as far as it 



