Archive image from page 158 of The development of the frog's. The development of the frog's egg; an introduction to experimental embryology . developmentoffro00morg Year: 1897 Cii. XIII] ORGAXS FRO:\I THE EXDODERM 143 that the whole embryo assumes. The early enlargement of the anterior archenteric cavity and the formation of a single- layered wall at the anterior end, with the subsequent formation of the gill-slits, would seem to be the result of the activity of the endodermal cells of those regions. On the other hand, some of the changes in shape that the lumen undergoes would seem to be due
Archive image from page 158 of The development of the frog's. The development of the frog's egg; an introduction to experimental embryology . developmentoffro00morg Year: 1897 Cii. XIII] ORGAXS FRO:\I THE EXDODERM 143 that the whole embryo assumes. The early enlargement of the anterior archenteric cavity and the formation of a single- layered wall at the anterior end, with the subsequent formation of the gill-slits, would seem to be the result of the activity of the endodermal cells of those regions. On the other hand, some of the changes in shape that the lumen undergoes would seem to be due to the change in shape of the whole embryo as it elongates antero-posteriorly, and narrows from side to side. Nevertheless, even in this case the cells do not seem to be Fig. 40. — Cross-section through the iiiiddle of an embryo (.'i mm.). AR. Archen- teron. jMs. Mesoblastic somites. X. Notochord. Ns. Neural crest. M. Nledul- lary tube. Pr. Pronephros. Sn. Subnotochordal rod. So, Sp. Somatic and splanchnic mesoderm. (After Mai'shall.) entirely passive, for the numljer of cells lining certain parts of the early archenteron is, in cross-section, considerably larger than the number lining the same region at a later stage. Either certain of the cells have pulled awav from the surface and have passed into the yolk, or else they have changed their position relative to one another on account of the lengthening of the archenteron. In the latter case the total number of endoderm cells lining the archenteron would still be the
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