No. 6.] DEVELOPMENT OF PARYPHA CROCEA. 293 



are borne upon long, slender, branched peduncles which arise a 

 short distance above the tentacles of the lower row. All parts 

 of the hydroid are made up of the two layers characteristic of 

 all hydroids, but the mesogloea forms only a thin layer in the 

 peduncles and tentacles and is not visible in the medusoids. 

 The tentacles of both rows consist of a central axis of the 

 endoderm, surrounded by a thin layer of ectoderm. 



Origin and MorpJiology of Male GonopJiores. 



The medusoids in this species begin to appear early in the 

 development of the hydroid, when the head upon which they 

 are borne is less than a quarter of the size of the adult polyp. 

 The first indication of their formation is a slight outpushing of 

 the endoderm of the body wall a little above the axils of the 

 lower row of tentacles. The ectoderm is pushed out and 

 becomes thinner than in the adjacent parts of the wall. The 

 papilla thus formed elongates into a peduncle communicat- 

 ing directly with the body-cavity of the polyp. From this 

 peduncle arise short branches which may subdivide, and it is at 

 the ends of these that the medusoids are borne. At first there 

 is merely a thickened layer of endoderm surrounded by a thin 

 layer of ectoderm, but when the length of the bud is about once 

 and a half the width, the ectoderm cells at the tip begin to 

 grow rapidly, forming a plug of cells with large nuclei and 

 indistinct boundaries (PI. I, Fig. i). For a time the endoderm 

 is forced back (PI. I, Fig. 2), but it soon begins to grow down 

 into the center of the plug, to form the manubrium, and around 

 the outside to form the endodermal layer of the bell. All the 

 cells between this layer and the manubrium are of ectodermal 

 origin, and from them the reproductive elements arise. The 

 sex cells increase in number, and to some extent in size, until 

 they occupy the greater part of the bell. While this growth 

 is taking place the cells at the distal end of the gonophore next 

 the endodermal layer of the bell begin to differentiate, forming 

 a thin, delicate layer which gradually extends around the gono- 

 phore and becomes the inner wall of the bell (PI. I, Fig. 3). 

 It is made up of cells much smaller than those from which they 



