No. 2.] THE HYPOPHYSIS OF A MI A CALVA. 65 



The characteristic stratification on the ventral side and the 

 irregular grouping of the cells above are still pronounced. 

 Very nearly in the center, between these lower strata and the 

 cells above, is a nearly spherical cavity (/.) about 16 /n in diam- 

 eter, the first lumen that has been recognized. This lumen is 

 not a longitudinal slit, as it is found only in this section ; and 

 since it is enveloped by no distinct membrane, and the cells 

 have no definite arrangement about it, it has the appearance of 

 an intercellular space. The mesoclerrn (vzs.) is seen to grow 

 between the brain and the mouth up to the sides of the hypoph- 

 ysis, just as at the ends. It would thus appear to be a 

 biconvex body fitting into concavities in the brain above and 

 the endoderm below. 



In a larva ten days old, of which Fig. 9 is a sagittal section 

 of the hypophysis and parts adjacent and Fig. 10 a transverse 

 section more highly magnified, the organ has become entirely 

 separated from the mouth roof, which has again become two 

 cells in thickness beneath it. A strand of mesoderm (MS.) 

 passes between the hypophysis and mouth roof and is continu- 

 ous at the sides with the thickened mesoderm differentiating 

 to form the cranial cartilages. An outfold of the infundibular 

 wall at its posterior lower margin, 33 //. by 39 ^ internal meas- 

 urement, is the first stage in the formation of the infundibular 

 gland, or saccus vasculosus (sv.). The single layer of cells char- 

 acterizing: the base of the infundibulum and the base of the thala- 



o 



mencephalon is continued into the saccus, but here the cells are 

 columnar, with nuclei at their outer ends, in marked contrast 

 with the cells in the brain wall contiguous. The chorda (c/i.) 

 runs forwards under the hind brain almost in contact with the 

 roof of the mouth, and its cephalic end (c/i/i.) abuts against this 

 infundibular process with a strand of mesoderm between. This 

 mesoderm grows into the space about the hypophysis, and a thin 

 strand of it runs between the organ and the mouth roof below. 

 The hypophysis now measures in longitudinal section 143 //. 

 by 57 P, in transverse 1 36 /A by 66 /A. A lumen may be seen in 

 the center, --an oval cavity with a membrane surrounding. 

 About this lumen the spindle-shaped cells seem to be arranged 

 in a radiate fashion, with long axes pointing towards it. 



