GEROULD: CAUDINA. 
61 
garded the micropyle as the stalk for the attachment of the egg. 
The observations of Ivdlliker (’58), which have been confirmed 
repeatedly by more recent investigators, made it necessary, however, 
to look for another explanation of the origin of this structure in 
holothurians, for he showed that the micropyle is situated at the pole 
of the egg opposite that at which the earlier observers had supposed 
it to be, viz., on the side next the lumen of the tubule. 
The other theory in regard to the origin of the micropyle was 
proposed by Semper (’68), who maintained that cells of the internal 
epithelium, constituting at first a single layer, retain throughout 
oogenesis their intimate connection with one another, constituting 
a single sheet. One of these cells, the fundament of an ovum, 
increases rapidly in size and lifts with it, as Semper supposes, a 
portion of the sheet of epithelium, which thus finally enwraps the 
growing ovum and becomes the follicle. By a constriction of the 
basal part of this elevation in the primitive sheet of cells, a sort 
of stalk is formed attaching the ovum to the ovarian wall; but the 
micropyle is established at the point directly opposite the stalk, 
where the ovum is supposed to retain its primitive connection 
with the original sheet of epithelial cells, a part of which has 
now become the follicle. Inasmuch, however, as the internal ovarian 
epithelium in Caudina does not, at the time when the future ova 
become differentiated, form a continuous sheet consisting of a 
single layer of cells, Semper’s hypothesis is entirely without foun¬ 
dation. 
An interesting abnormal condition is shown in Plate 7 , fig. 98, 
which is a surface view of a well-grown ovarian ovum with four 
micropyles. The zona radiata was much swollen, from having 
remained for twenty hours in an aqueous stain. The eggs of the 
lot from which this specimen was taken possessed quite uniformly 
four micropylar structures, never more, though in some cases less 
than that number. 
After the formation of the follicle and zona radiata and the 
establishment of the micropyle, the ovum continues to grow by 
assimilation of nutriment derived from the homogeneous blood 
fluid in the longitudinal lacunae, which fill the lumen of the tubule 
and come into close contact with the ova. The nucleoli, the 
greater number of which are in contact with the nuclear membrane 
