32 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



Andersson did not find more than 5 embryos in a marsupium, generally 

 only 3. I have, however, found as many as 8 eggs and embryos together 

 in one marsupium. 



The fertilization probably takes place within the ovary itself. Andersson 

 discovered the remarkable fact that there is a space filled with spermatozoa 

 in each ovary, from which it must evidently be concluded that a sort of copu- 

 lation must take place in this species. It can scarcely be doubted that the 

 fertilization then must take place within the ovary, before the eggs are 

 emptied into the marsupium. It may, however, be stated that I have failed 

 to find any fertilized egg within an ovary, nor does it appear that Andersson 

 has found this. On the other hand, I have not found any unfertilized egg 

 in the marsupia. 



The egg-membrane is very thin and quite simple, without any special struc- 

 ture (plate xiv, figure 1). The embryos remain within the membrane during 

 their whole development and do not leave it until they leave the marsupium 

 as fully formed larvae. In the figures the membrane has been omitted, except 

 in the very youngest stage of development (plate xiv, figure 1). 



The cleavage is very remarkable and thus far unique among Crinoids 

 (plate xiv, figures 1 to 3), corresponding to the superficial cleavage typical 

 in Arthropods. The egg does not divide at all; only the nuclei divide and 

 spread irregularly in the yolk-mass. In the youngest stage found some few 

 nuclei are seen lying in the middle and a few near the surface (plate xiv, 

 figure 1). Gradually they multiply, and then a number of nuclei are found 

 lying irregularly spread in the mass of yolk with no trace of cell limits (plate 

 xiv, figures 2 and 3). The nuclei are here beginning to arrange themselves 

 along the surface, while another, less distinct, group occupies the middle of 

 the embryo. Whether an actual wandering of the nuclei towards the surface 

 takes place, or whether the outer nuclei descend exclusively from those lying 

 near the surface in the youngest stage, the inner ones from those lying in the 

 middle, can not be determined. 



The next stage is represented in plate xiv, figures 4 and 5. Here the 

 ectoderm and entoderm have been differentiated. The nuclei at the surface 

 now form a fairly regular layer, and cell limits have begun to appear, 

 so that there is now a distinct ectoderm of typical form, clearly delimited 

 from the entoderm. The cells are high, cylindrical, and full of round yolk- 

 grains (plate xv, figure 3). The nuclei in the middle of the embryo have 

 collected in a more definite group and in the middle of this group the archen- 

 teron begins to appear as a small, narrow slit (plate xiv, figure 5). It soon 

 grows in size and the nuclei range themselves more regularly around it; 

 then cell limits begin to appear and thus the entoderm is formed; accordingly, 

 there is no blastoporus, as no invagination takes place (plate xiv, figure 4). 



It is interesting to notice that there is some inconsistency in the order of 

 appearance of the developmental process, as appears from a comparison of 



