194 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 
3. Another way in which the mass changes form without growth is 
by vacuolization. "Through vacuolization there arise, e. g., the blastula in 
many eggs, the secondary body cavity in many Invertebrates (K. & H. 
Fig. 131, 689), and in Bryozoa the cavities of the bud. This process of 
vacuolization, which I introduce for convenience at this place, is not 
confined to masses. Many cords by vacuolization (canalization) become 
tubules, and many layers become divided into two. Of vacuolated cords, 
I need refer only to the formation of capillaries in Vertebrates and of 
nephridia in Invertebrates; of vacuolated layers, to the origin of the 
cœlom in most Vertebrates. In all cases, the acquisition of a mass 
of water at the centre causes a rearrangement of the nuclei. 
4. Perhaps this is the most fitting place to mention the process of 
Dispersion of elements, which occurs not only in bodies originally 
formed by aggregations of mesenchymatous elements (Fig. 28), but 
also in layers having an epithelial origin, e. g. the ectoderm of Dis- 
tomum. (K. & H., Fig. 88.) 
IV*. The remaining processes occurring in protoplasmic masses are of 
such a nature as to alter the number 
\ ) of masses. We can distinguish, corre- 
4 spondingly, two classes: the first in- 
BER secondary cluding such as have to do with the 
formation of two masses from one, 
through division ; the second including 
those which have to do with the union 
of two masses into one. The first, 
then, are division processes; the sec- 
ond, fusion processes. 
5. Under the first head we may in- 
| ONUS elude the process of constriction, by 
ug. emevye which two more or less independent 
N masses arise from one. An illustra- 
tion of this is found in the case 
of embryonic fission described for 
some Bryozoa by Harmer (Figure 29). 
Closely allied to this is the process of 
sloughing off of a part of the body’ in 
metamorphosis, one of the most striking instances of which is shown in 
Fig. 29. Section through a brood chamber of Crisea eburnea, showing the con- 
stricting off of secondary embryos from the primary embryo. After Harmer, ’93, 
Quart. Jour. Mic, Sci, XXXIV., Plate XXIII. Fig. 11. 
