50 J. B. Johnston. 
extending in from the deeper layer of the ectoderm. A slight cavity 
is 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 pushed 
back, flattened, and its cavity nearly obliterated. This cavity be- 
comes less prominent before the ‘closing up of the entodermal mouth 
Via. 8. A. punctatum, nearly median sagittal section to show the persistence 
of the preoral or paleeostomal recess in the entoderm. The invagination of 
the hypophysis is very tardy in this specimen. THeemalum. 
pit. There is always to be seen, however, a shallow pit or an angular 
prolongation of the archenteron which continues to indicate the 
preoral cavity (Figs. 8, 9, 10). In some eases a cleft appears in 
the entoderm leading toward the cavity of the hypophysis. By this 
time the entoderm has been compressed and thickened, but the per- 
