MUSEUM OF COMPARATIVE ZOÖLOGY. 9 
nuclei are already arranged in a deep saucer-shaped layer. The transi- 
tion to the U-shaped arrangement of Figure 37 (Plate IV.), in which 
the invagination of the inner layer of the bud is completed, is not a diffi- 
cult one to understand. It is to. be observed, however, that the folding 
is of such character that it can hardly be termed a typical invagination. 
Comparing Figures 4, 5, and 37, it appears rather to be of a type some- 
what intermediate between typical invagination and typical ingression. 
The cavity of the bud first arises through a rearrangement and reshap- 
ing of the cells of the inner layer of the bud. At this stage the nuclei 
of the invaginated region stain very deeply, and have large nucleoli. 
Figure 21 (Plate III.) shows the condition of the bud at this stage as 
seen in longitudinal section. The proliferation which gave rise to the 
rudiment of the bud is shown, by a comparison of Figures 37 and 21, 
not to have been confined to one point, but to have occurred along a line, 
so that the resulting bud is boat-shaped, and not cup-shaped. The whole 
mass is therefore bilaterally symmetrical. Even at this early stage one 
can distinguish a difference in the form of the bud at the anal and oral 
ends. At the oral end (Or.) the bud passes more abruptly into the body 
wall than at the anal end. Later, this feature becomes more marked. 
This is an indication of a fact for which later stages will bring better 
evidence; that the formation of the bud proceeds from the oral towards 
the anal end, and that the increased length of the bud that one finds in 
the stage represented by Figure 22 is due to growth at the anal end. 
4. ORIGIN AND DEVELOPMENT or THE LATERAL BRANCHES. 
The first lateral branch appears as a prominent protrusion of the lat- 
eral walls of an individual of the primary branch when the ganglion of 
that individual has already nearly closed, and when the bud of the next 
younger individual has attained a stage somewhat later than that shown 
in Figure 37. The zone in which the lateral buds arise lies about mid- 
way between the neck of the median polypide and the tips of its tenta- 
cles at this stage. The place of appearance in this zone is approximately 
90° to the right or left of the neck of the polypide of the median indi- 
vidual, In one case measured, however, that shown in Figure 20 (Plate 
II.), the centres of the two lateral buds seemed to be unequally distant 
from the neck of the polypide, and each over 90° from it (approximately 
100° and 110° respectively. (Compare page 3.) 
A cross section of the branch through the region in which the lateral 
bud is arising shows that the condition of the body wall at the bud is 
quite different from that of the rest of its extent, Figure 19 represents 
