The American journal of anatomy . tion (o show the persistenceof the preoral or palseostomal recess in the entoderm. The invagination ofthe hypophysis is very tardy in (his specimen. Hsemalum. pit. There is always to be seen, however, a shallow pit or an angularprolongation of the archenteron which continues to indicate thepreoral cavity (Figs. 8, 9, 10). In some cases a cleft appears inthe entoderm Lading toward the cavity of the hypophysis. By thistime the entoderm has been compressed and thickened, bnt the per- The Limit Between Ectoderm and Entoderm. 51 sistence of the preoral cavity (in v
The American journal of anatomy . tion (o show the persistenceof the preoral or palseostomal recess in the entoderm. The invagination ofthe hypophysis is very tardy in (his specimen. Hsemalum. pit. There is always to be seen, however, a shallow pit or an angularprolongation of the archenteron which continues to indicate thepreoral cavity (Figs. 8, 9, 10). In some cases a cleft appears inthe entoderm Lading toward the cavity of the hypophysis. By thistime the entoderm has been compressed and thickened, bnt the per- The Limit Between Ectoderm and Entoderm. 51 sistence of the preoral cavity (in various degrees) serves to markthe position of the preoral entoderm. It also explains the peculiarform taken by the hypophysis. The cleft meeting the cavity of thehypophysis suggests that the intact Avail of the hypophysial invagina-tion constitutes, with the entoderm dorsal to the cleft, the dorsalwall or roof of the palseostoma, while the absence of a ventral wallto the hypophysial invagination allows the hypophysis to open into. Fig. 9. A. punctatum, after separation of hypophysis from sagittal section. The pointed recess of the archenteron directed towardthe letter m is the oral recess; the slight pit or concavity next to pr. en, isthe palaeostomal recess. Haemalum. the archenteron, the ventral wall or floor of the palseostoma beingformed by entoderm alone. These relations are indicated in adiagram (Fig. 11). After the stage at which the partial connection between hypophysisand archenteron is seen, there is a further bending of the brain, theflexure becoming excessive in amphibians. The bending, togetherwith the expansion of the forebrain, results in pushing the hypophysis .1. II. Johnston. and preoral entodi rm farther and farther back, until they cometo lie close to the end of the notochord. Compare Figs. 3 and is this movement of the preoral entoderm dorso-caudally that wasreferred to above as the means of thickening up the wall of the archen-teron wh
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1910
