636 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



between 6 and 7 and on the last between 12 and 13. Finally, the side plates of the 

 pinnule ambulacra, which are well developed together with the cover plates, are plate- 

 like rather than rodlike. There are few spicules in the tentacles. The first two 

 pinnules have about the same relative lengths as in most specimens of vivipara, with 

 P, about 0.5 mm. longer than P 2 , but PI is very little stouter at the base (see fig. 36, 6 

 and c). 



This specimen is the first one at the top of table 16 and is the one marked x hi 

 the scatter diagram on page 835 illustrating the relative proportions of the first two 

 pinnules. In the key to the species of Isometra it runs down to /. flavescens, known 

 from the Shag Rocks, but differs from that species in the smaller size difference be- 

 tween the first two pinnules (shown clearly in the graph) and the more complex am- 

 bulacral deposits. 



More material from the Bransfield Strait area is needed before a definite conclusion 

 on the identity of this specimen can be reached. 



Early stages. The eggs are about 0.3 mm. in diameter, very rich in yolk, and quite 

 opaque. They are found in various stages of development in the ovary, and are liber- 

 ated as they become ripe and not all at once as is usually the case in the crinoids. 

 Thus one may find fully formed larvae together with embryos in the first cleavage 

 stages in the same marsupium. Andersson did not find more than 5 embryos in a 

 marsupium, and generally only 3; Mortensen, however, found as many as 8 eggs and 

 embryos together, while John (1938) found up to 6 eggs and 21 embryos in a single 

 marsupium. 



Andersson found that in each ovary there is a space filled with spermatozoa, and 

 Mortensen concluded from this that fertilization probably takes place within the 

 ovary itself. But neither he nor Andersson found any fertilized eggs in the ovary, nor 

 did he find any unfertilized eggs in the marsupia. 



The egg membrane is very thin and quite simple, without any special structure. 

 The embryos remain within the membrane during the whole course of their develop- 

 ment and do not escape from it until they leave the marsupium as fully formed larvae. 



The cleavage corresponds, according to Mortensen, to the superficial type charac- 

 teristic of arthropods. The egg as a whole does not divide at all; only the nuclei 

 divide and scatter irregularly in the yolk mass. In the youngest stage found, a few 

 nuclei are seen lying in the middle and a few near the surface. These gradually multiply 

 so that a number of nuclei are found irregularly scattered in the mass of yolk with no 

 traces of cell walls. The nuclei arrange themselves about the periphery, with another 

 group, less distinct, in the middle of the embryo. Mortensen could not determine 

 whether an actual wandering outward of the nuclei takes place, or whether the periph- 

 eral nuclei are derived exclusively from those lying near the surface in the youngest 

 stages and the inner from those lying in the middle. 



In the next stage represented the superficial nuclei form a fairly regular layer and 

 cell limits have begun to appear so that there is now a distinct typical ectoderm of high 

 cylindrical cells, full of rounded yolk granules clearly delimited from the entoderm. 

 The nuclei in the middle of the embryo have assembled in a more definite group, and in 

 the center of this group the archenteron appears as a small narrow slit, which soon grows 

 in size, the nuclei arranging themselves more regularly around it and cell limits appear- 

 ing. An important differentiation is beginning to take place in the yolk substance. 

 While the whole of the entoderm and ectoderm is uniformly filled with yolk granules, 



