DEVELOPMENT OF DIGITIFORM GLAND 337 



one-half as wide as the radius of the gland, contains radial tubules 

 of the second, third, fourth and fifth, but mostly of the third 

 and fourth order. The peripheral two-sevenths of the gland 

 contains tubules of all six orders but mainly of the fifth and 

 sixth. At the periphery some of them end in points, some are 

 dilated and others bend to run a short distance longitudinally. 

 Most of the tubules in the adult gland are about 60 /x in diameter 

 except at a junction of two or more tubules or at dilated 

 extremities. 



Cytology. The earliest stage drawn (fig. 1, 15 mm.) shows 

 no cellular changes in the area of evagination as compared with 

 the rest of the post-valvular intestine, beyond a slight conden- 

 sation of nuclei, and a slight rounding of many of them. Cell 

 boundaries are indistinct, but four or five rows of nuclei may be 

 counted. The surrounding mesenchyma contains more lightly 

 staining oval nuclei and in the serosa the nuclei are dark and 

 nearly spherical. A few endothelial lined vessels are seen in 

 the mesenchyma. 



In the anlage of the digitiform gland in the 19 mm. embryo 

 (fig. 2) many of the nuclei are round and show change of posi- 

 tion. The longest axis of main' of the elongated nuclei is paral- 

 lel with that of the intestine. The nuclei in this stage are rela- 

 tively fewer in number than in the 15 mm. stage, but are of 

 about the same size. The lateral bulging of the wall of the gut 

 has flattened the mesenchymal cells covering the anlage of the 

 gland. The mesothelium is likewise flattened in this region, its 

 nuclei being oval in shape. Vascular channels are numerous. 



In the 22.5 mm. stage the epithelium of the digitiform gland 

 is still composed of four layers of cells as shown by the nuclei, 

 the cell boundaries being indistinct. At the rapidly growing, 

 distal end of the gland, most of the nuclei are round, but toward 

 the gut they appear oval in outline. The anlage is surrounded 

 by flattened mesenchymal cells covered with a thin peritoneal 

 layer. The lumen is still smooth and regular. 



As shown by figures 3 and 7 of an Acanthias embryo of 28.1 

 mm. tubule formation is well under way. These evaginations 

 are relatively very large, involving often as much as a fifth of 



