The American journal of anatomy . ^ : Fig. 6. A. punctatum, in a stage slightly advanced over that in Fig. sagittal section of hypophysis. Borax carmine. Fig. 7. A. punctatum, at the height of the hypophysial invagination beforeseparation from ectoderm. Note the concavity of hypophysis and the absenceof ectoderm from the month plate. Hsemalum. below). As the hypophysis pushes in, its thin posterior or ventralwall is devoid of cells, and soon the cavity of the hypophysial invagi-nation is bounded on that side by entoderm (Figs. 6 and 7). Thehypophysis appears as a hook-shaped membrane
The American journal of anatomy . ^ : Fig. 6. A. punctatum, in a stage slightly advanced over that in Fig. sagittal section of hypophysis. Borax carmine. Fig. 7. A. punctatum, at the height of the hypophysial invagination beforeseparation from ectoderm. Note the concavity of hypophysis and the absenceof ectoderm from the month plate. Hsemalum. below). As the hypophysis pushes in, its thin posterior or ventralwall is devoid of cells, and soon the cavity of the hypophysial invagi-nation is bounded on that side by entoderm (Figs. 6 and 7). Thehypophysis appears as a hook-shaped membrane (sagittal section) 50 ,T. I!. Johnston. extending in from the deeper Layer of the A slight cavityis contained within the recurved wall at the deeper end (Fig. 7),the rest of the ventral wall being formed by entoderm. While the hypophysis is pushing in, the preoral entoderm is pushedback, flattened, and its cavity nearly obliterated. This cavity be-r, iines less prominenl before the closing up of the entodermal month. Vic. S. A. punctatum, nearly median sagittal section (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
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1910
