8 BULLETIN S2, UNITED STATES NATIONAL MUSEUM VOLUME 1 



3 embryos were found associated in a marsupium, and generally there were only 1 or 2. 

 Mortcnscn says these facts would seem to indicate that some of the eggs do not develop. 

 In some cases he found in marsupia without embryos a yellow coarsely granulated 

 substance which had the appearance of being eggs in process of disintegration. This 

 suggested that some of the eggs are destined to serve as nourishment for the developing 

 embryos. The unusual size of the embryos would be accounted for by this suggestion, 

 while the size of the eggs cannot explain it since their size is by no means unusual, not 

 larger than in Isometra vieipara. 



All the embryos found are in nearly the same stage of development. Although 

 they occur in various sizes there is no essential difference in the developmental stages 

 represented. Only in a single instance Mortensen found the vestibular invagination 

 in a much j'ounger stage than usual, being represented only by a slight concavity, along 

 the ventral side, in which the ectoderm is considerably thickened. Internally except 

 for a lesser development of the glandular sacs (mentioned beyond) the development of 

 this larva is as advanced as that of the others. 



Mortensen saj's that the occurrence of the same developmental stage in all the 

 embr3'os would seem to indicate that the eggs are passed into the marsupia a number 

 at a time, not one at a time and at any time as in Isometra vivipara. 



He believes that this peculiarity in Isometra vivipara is correlated with the fact 

 that in that species the spermatozoa are collected in a sort of vesicula seminalis in the 

 ovary, perhaps through copulation, and are always available for fertilizing the ova, 

 whereas in all other crinoids, so far as known, the eggs are extruded only when a male 

 emits sperm, the sperm seemingly acting as a stimulus. 



All the larvae are almost fully formed. The smallest are 0.9 mm., the largest 1.8 

 mm. in length. Evidently the egg membrane was ruptured a long time before the 

 embryos reached that size. In embryos sectioned in position within the marsupia, 

 parts of what would appear to be an outer membrane are seen close to the sldn, but 

 otherwise there is no trace of an egg membrane to be observed. Mortensen says that 

 this accords well with the fact that the larvae grow to such large size that they must 

 perforce be assumed to obtain nourishment from the mother; this nourishment could 

 hardly pass through an egg membrane and, as there is no mouth, it must be taken in 

 by absorption thi-ough the skin. Mortensen noted that in embedding the larvae in 

 paraffin the skin would always break in various places in spite of the most careful 

 treatment; but when they were embedded lying undisturbed within the marsupia the 

 skin never broke. 



The form of the larva is generally more or less irregular because of the pressure in 

 the marsupium, but generally it is somewhat flattened and slightly concave on the 

 ventral side and dorsally more arched, posteriorly more so than anteriorly. On the 

 ventral side at the anterior end is a more or less distinct arcuate depression with the 

 convexity directed anteriorly. This represents the suctorial disk. There is no apical 

 pit- 



The vestibulary invagination is narrow. In those larvae apparently most normal 

 in shape is a shouldcrlike prominence on either side at the anterior end, but Mortensen 

 could not determine whether or not this is a typical feature. 



In one larva there was a slender prominence like a thin stalk on one side, suggesting 

 that it had been attached to the wall of the marsupium, but as nothmg similar was 

 observed in any other embryo Mortensen says this carmot be a normal feature. 



