MONOGRAPH OF THE EXISTING CKINOIDS. 365 



centrosome has been crowded out of the socket in the anterior end of the nucleus. 

 Field regards this membrane as the original cell membrane which has persisted 

 from the spermatid. 



GENITAL ORGANS OF ISOMBTRA VIVIPARA. 



The female genital organs of Isometra vivipara have been described by Dr. 

 K. A. Andersson, and to Andersson's description Dr. Th. Mortensen has added a 

 few supplementary observations. 



A cross section through a fully developed female pinnule of this species in the 

 region covered by the expanded third to fifth pinnulars (to which region the 

 development of the ovary and of the accessory structures is confined) shows two 

 striking peculiarities : On the distal side a deep and narrow pocket with a short and 

 narrow slit-like opening situated close to the ambulacral groove shows the presence 

 of a brooding chamber or marsupium, which is separated from the ovary by a very 

 thin membrane containing no calcareous deposits, and in the center of the ovary 

 there is a special cavity for the reception and conservation of the spermatozoa a 

 receptaculum seminis. 



Except for a small canal occasionally narrowing to a small slit, which extends 

 throughout its entire length, the ovary is an almost solid structure. This canal 

 represents the genital tube of other crinoids, but, instead of being filled with eggs, 

 it contains a variable number of spermatozoa and evidently functions as a recep- 

 taculum seminis. It is provided with longitudinal evaginations which extend in 

 between the eggs, so that on a cross section it appears as an irregular or star-shaped 

 figure in the middle of the ovary. It reaches the outer border of the latter only in 

 a very limited area, where the ovarial wall is thinnest, between the ovary and the 

 marsupium, being here separated from the latter only by a single layer of cells. 



The wall of the receptaculum seminis is composed of a single layer of cells, 

 which forms the germinal epithelium. 



In young ovaries, or in slightly developed portions (as, for instance, the 

 proximal portion) of mature ovaries, some of the cells of the germinal epithelial 

 layer are seen to be larger than the others and to be provided with large nuclei. 

 These cells, which arise from the outer wall of the receptaculum seminis (repre- 

 senting the genital tube of other crinoids) , are small egg cells. 



In young ovaries many of the cells in the germinal epithelium are in process 

 of division; between these are smaller cells, which sometimes are provided with 

 long processes, which are not egg cells; in fully developed ovaries these cells are 

 greatly in excess and the egg cells are very few. 



Beyond the layer of cells just described is a thick layer of connective tissue, in 

 which the large eggs are embedded, and between the ovary and the genital sinus 

 is a thin layer of pavement epithelium. There is no genital cavity, as in Antedon, 

 Heliometra, Tropiometra, and the other comatulids which have been studied. 



As the egg cells increase in size they migrate, not into the cavity of the genital 

 tube (which in this case has become the receptaculum seminis), as in other forms, 

 but into its wall, into which they gradually sink deeper and deeper. They never 

 entirely sever their connection with this cell layer. 



