192 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
pockets, like the lens and otocysts, become transformed into closed SACS ; 
and grooves, like those of the lateral line, become transformed into canals. 
In Invertebrates also this process is a very common one, being exempli- 
fied in the closure of the blastopore, in the closure of the amniotic cavity 
in Insects (K. & H., Figs. 474, 475, 484), and in the formation of optic 
and otic vesicles generally (K. & H., Figs. 377, 630, 681-683). 
The end of the process of concrescence proper is a fusion of the two 
concrescing layers, whether the concrescence is occurring along free 
edges, flatwise, or along the edges of folds. (Fig. 25, C, Fig. 24, IL) 
9. Frequently this process is followed by another one; viz. the per- 
foration of the fused layers (Fig. 24, III), or the separation of the 
ıpper and lower components of the folds when two folds have been 
concrescing (Fig. 25, D). By means of perforation the two spaces 
separated by the fused walls are put into communication with one 
another. Dy means of separation a pocket becomes a closed sac, and 
a groove becomes a tube, 
IV. ONTOGENETIC Processes OCCURRING IN PrororLasmıo MASSES. 
These may be classed into three categories according as the most 
v o I] 
rominent change produced is (IV*) in volume, (IV?) in form, or 
’ , 
(IV*) in number of masses. 
IV® 1. Under the first group are included changes produced by 
growth which is not uniform in all parts. Thus the growth may be 
prevailingly along one axis, by which 
means a cylindrical mass is derived 
Spinal cord. 
from a spherical one (embryos of Dy- 
ciemide, K. € H., Fig. 99), or it may 
be excessive at one pole (gemmules of 
sponges, H. V. Wilson, '94, Jour. of 
Morphol., IX., Pl. XVL), or along one 
meridian. Again the growth may be 
more localized, being confined to a 
small area or to a line; as, for in- 
Fic. 26. l stance, in the case of the mesodermal 
core of the appendages of Arthropods 
(K. & H., Fig. 371), and of Vertebrates (Figure 26, “ limb-bud ”). By 
ugh embryo of a Teleost, Fundulus, showing origin 
of the pectoral limb-bud as a solid outgrowth of the somatopleure. After E. R. 
Boyer, ’92, Bull. Mus. Comp. Zoöl., XXIII No. 2, Fig. 58. 
Fig. 26. Cross section thro 
