138 
Transactions of the Society. 
The pronuclei have numerous small plasmosomes, and during fusion 
of the gametes, the plasmosomes come together to form a large 
spherical nucleolus. 
The case of the sponge worked out by Gatenby (12) is similar. 
Here the sperm is carried to the egg by a modified collar cell. 
Before entering the egg it has a single nucleolus, but on entering 
the egg the whole cell begins to grow and other nucleoli appear. 
The female pronucleus also has numerous nucleoli, and well-marked 
nucleoli are present during segmentation of the ovum. 
During artificial parthenogenesis of the egg of the sea-urchin 
Strongylocentrotus purpuratus, changes have been observed in the 
nucleolus. Jenkinson (29), quoting the observations of Hindle 
(26), says that well-marked changes occur in from fifteen to 
twenty minutes between removal from the butyric acid and 
immersion in the hypertonic salt solution. The fertilization 
membrane of the egg is first thrown off, then the nucleolus loses its 
affinity for dyes, and also its shape. It may fragment. In the 
hypertonic salt solution the nucleus enlarges, and this is still 
more marked on transferring again to sea-water. These observa- 
tions point to the probability of the variations in nucleolar number 
and behaviour being due to different metabolic conditions in the 
nuclei. 
Explanation of Plate V. 
Fig. 17. — Another section through a blastula, showing a binucleate cell at BC, 
chromosomes at metaphase at CH, and the disintegration of the nucleolus (NY) 
preparatory to mitosis. 
Fig. 18. — Two other nuclei from the same blastula. At “ a ” another double 
nucleus ; at 11 v ” nucleus showing fragmentation of the nucleolus (NL). 
Fig. 19. — Mesoderm cell of gastrula, showing the large amount of nucleolar 
material (NL) and possible extrusion. 
Fig. 20. — Two mesoderm cells recently separated from the endoderm, to 
show the growth of the nucleolus. 
Fig. 21. — Another mesoderm cell with large nucleolus, which is surrounded by 
a peripheral layer of deep staining substance, inside of which is a fainter staining 
material ; pseudopodia-like process at NP, and possible extrusion at NX. 
Fig. 22. — Two endoderm cells at early gastrula stage. The cytoplasm of each 
contains several vacuoles. At NX is shown what is believed to be a nucleolar 
extrusion. 
Fig. 23. — Endoderm cell of the trochosphere larva, showing the large vacuole 
(V) and the considerable growth which has taken place in nucleus and nucleolus. 
Fig. 24. — Section of larva at the region of invagination of the stomodseum. 
Cell at prophase shown at CP, another at late telophase at CT. Possible nucleolar 
extrusions denoted by NX. 
Fig. 25. — Part of wall of larva, showing a good example of nucleolar activity 
at ET. 
Fig. 26. — Ectodermal (ET) and mesenchyme (ME) cells of trochosphere larva, 
showing nucleolar activity and probable extrusions. 
Fig. 27. — Part of the ciliated epithelium of a duct of the digestive gland, 
showing the relatively inactive nucleoli (NL). 
Fig. 28. — Secreting cells of the digestive gland, showing the vacuolated 
nucleoli with pseudopodia-like processes (NP). At NX possible nucleolar 
extrusion. 
