358 E. R. HOSKINS 



solid gut in this stage is much the same as that in the 95 mm. 

 embryo. Each ridge shows four to six rows of elongated nuclei 

 at the periphery, the two opposite halves being connected by 

 central cells, irregular in form, with elongated nuclei lying at 

 right angles to those of the peripheral cells. Toward the two 

 ends of the solid region the small isolated lacunae in the epi- 

 thelium are more numerous than in the preceding stage and in 

 places some have united longitudinally, so that the solid epi- 

 thelium of each ridge is beginning to split. This process must 

 be completed shortly after this stage because a 200 mm. Acan- 

 thias is almost full term. The increase in the circumference of 

 the lumen thus established, as may be noted by comparison of 

 figures 15, 19, 20 and 21, is many times greater than the increase 

 in the length of this portion of the gut, and thus possesses a 

 surface area not much less than a segment of the spiral valve 

 of equal length. 



Summary of part 2 



The post- valvular portion of the intestine in Acanthias in 

 young stages is a simple tube. Various markings in the form of 

 irregular longitudinal ridges and depressions soon appear on the 

 external surface of the gut. The ridges increase rapidly in 

 height. As the spiral valve twists, these ridges turn in the 

 same direction. 



The gut loses its circular outline (as seen in transverse sec- 

 tions) and becomes much flattened, especially in the region of 

 the digitiform gland. Owing to a rapid longitudinal growth, 

 the gut becomes bent in this region into an S form. A dorsal 

 portion of the cloaca into which the Wolffian ducts early enter 

 becomes marked off from the rest of this chamber to form a 

 bladder. 



The lumen of the gut in the region just anterior to the duct 

 of the digitiform gland becomes occluded by a rapid prolifera- 

 tion of the cells in the center. These cells produce a centrifugal 

 force which accounts largely for the ridge formation. The rela- 

 tively thick solid plug of epithelium soon becomes almost com- 

 pletely divided into solid ridges which in turn are subdivided 



