264 BULLETIN OF THE 



clearly to be seen through the side of the hydrophyllium, and the en- 

 veloping layers of the yolk are traceable over its whole surface. Within 

 the segmented yolk cells appears the protoplasmic network {yt. c.) which 

 dates back to the original ovum in the gonophore. The primitive hydro- 

 phyllium is seen fitting over the ovum like a helmet, which, although 

 fastened to it at the germinative pole, is free on the sides. Its border 

 and sides cover about two thirds of the yolk which is here represented 

 through the transparent lateral walls. 



The primitive hydrophyllium is transparent, slightly reddish in cer- 

 tain regions, its great bulk being gelatinous. The following distinc- 

 tion between the distal and proximal rim can be easily seen when in 

 profile. The wall of the distal edge, which corresponds to the visor of 

 the helmet of our former comparisons, is much thicker than the opposite, 

 and more rounded. The proximal rim ends in a sharp angle, and its 

 walls are very thin. In a figure of this stage we have the larva repre- 

 sented as if we were looking at it from the left-hand side as defined 

 above. Over the surface of the primitive hydrophyllium is spread a 

 single layer of thin polygonal cells of the epiblast, which are seen in 

 profile around the rim of the bell and on its bounding lines, even on 

 the inner surface adjoining the yolk sac. Over the external surface 

 the prominent nuclei of the same cells can be readily traced, dotting it 

 at intervals, and in places well-defined cell-walls can be faintly seen. 

 The layer from which these cells came, or the epiblast, was one of the 

 first layers to form, and throughout the growth it has been gradually 

 becoming relatively thinner and thinner. Although the bodies called 

 nuclei of these cells are very well marked in Agalma elegans, I do not 

 find them represented in the figures which have been published of other 

 species of the genus Agalma, or Cri/staUodes. The remnant of that 

 cavity, which has been called the primitive cavity, "is now a tubular body 

 with thickened hypoblastic walls of yellow color, and extends froin the 

 base of the float towards the distal portion of the hydrophyllium. 



At this stage in the development of the primitive hydrophyllium it 

 was observed that from the nuclei of several of the epiblastic cells, situ- 

 ated on the surface of the hydrophyllium above the fundus of the 

 cavity, there were thread-like extensions, probably protoplasmic, which 

 connect the surface of the larva with the hypoblast of the cavity. At 

 times the surface of the hydrophyllium from which these threads arise 

 is depressed as if forcibly drawn back by them. In a few instances the 

 threads end blindly in the gelatinous layer at a point not more than 

 half-way from the epiblast to ♦^he cavity. These threads sometimes 



