The Morphology and Physiology of the Nucleolus. 137 
swimming trochospliere. In fig. 25 is an ectodermal cell (ET) in 
which the nucleolus has fragmented, and this seems an almost 
definite case of nucleolar extrusion. No doubt such a condition 
arises by the breaking up of a nucleolus, the same as that shown 
in the adjoining ectodermal cell. The metaphase of the meso- 
dermal cell ME, is another example where no nucleolus is visible 
during cell division. 
In fig. 26 are drawn four ectodermal and three mesenchyme 
cells. The nucleolus in one of the ectodermal cells is single and 
spherical ; in two of the cells, one ectodermal and one mesodermal, 
it is seen in process of division (DN). The problem arises here 
as to whether a portion of the nucleolus is extruded into the 
cytoplasm. The position of the granules NX seems to indicate 
that parts of the nucleolus may be thrown out of the nucleus into 
the cytoplasm, where they diffuse. 
Portions of the tissues of the adult animal are shown in 
figs. 27 and 28. The former represents the ciliated epithelium of 
one of the ducts of the digestive gland ; the latter some of the 
secretory cells of the same glands. The feature of importance is 
the relative inactivity of the nucleolus in the epithelial tissue 
compared with that of the secreting cells. No nucleolar extrusion 
is apparent in the first case, but there is strong evidence for 
regarding the granules NX in the gland cells as of this nature. 
Discussion. 
(a) Number of Nucleoli during Gametogenesis and Segmenta- 
tion . — There is remarkable variation in the number of nucleoli 
present in the nucleus of animal cells during gametogenesis and 
segmentation. In Limnsea I have observed no definite nucleolus 
in either the pronuclei or the early segmentation stages of the 
ovum. Griffin {18), who worked out the maturation, fertilization 
and cleavage of the Echiuroid Thalassema and the Lamellibranch 
Zirphtea, also does not figure any nucleoli at the same stages. The 
case of the mollusc Zirphsea is specially interesting in this 
connexion because a double nucleolus is present in the oocyte 
similar to that which I have described. Hartmann (##), too, who 
worked on the opossum, Didelphys virgiana, makes no mention of 
a definite nucleolus in the gamete nuclei, but the nucleolus during 
embryogeny is well marked. Attention is drawn by Hartmann 
to the vacuolation of the cytoplasm of the ovurn, such as I have 
described and figured. 
In contrast to these observations we have cases where the 
nucleolus is well marked in both gametes. Walton (60) describes 
in Ascaris canis that, preparatory to the first maturation division, 
the plasmosome of the oocyte nucleus degenerates and disappears 
in situ, some of its elements being extruded into the cytoplasm. 
