308 PROCEEDINGS OF THE ACADEMY OF [April, 
anterior quadrants four cells of equal size are formed in each quadrant, 
and as the blastopore continues to narrow these cells migrate as a 
group in each of the two anterior quadrants, approaching the blasto- 
pore and slipping over the cells 3b’ and 3b”, 3a*" and 3a”, which lie 
between them and the smaller cells of the same series (Pl. XXIX, 
figs. 68, 69). During this period the third quartet blastomeres of the 
posterior quadrants remain as before. 
The blastopore thus becomes entirely surrounded by the second and 
third quartet elements, of which the third are much more numerous, 
having the small cells 2a”-2d” or their derivatives wedged in between 
them on the median and transverse line. The gastrula, taken as a 
whole, is much flattened dorso-ventrally and is at first shorter in its 
longitudinal than transverse axis. The blastopore assumes a slit-like 
form, its longitudinal axis corresponding to the future longitudinal 
axis of the embryo. 
The next important change to be observed is the origin of the 
Ecto-Mesoblast. 
As the cells 3at!, %, 1, “ and 3b™, 17, “1 contmue to move 
toward the blastopore, the cells which they are covering over 3a”, 
3a24 and 3b2!!, 3b’, sink downward into the segmentation cavity. 
As this occurs they all four divide, giving rise externally and in the 
direction of the blastopore to four small cells, 3a”, 3a”” and 3b”, 
3b”, while the larger daughter cells continue to retreat beneath 
the overgrowing ectoderm (fig. 74). These larger cells, 3a", 3a”, 
3b24t and 3b%4, are the source from which the secondary mesoderm 
is derived. They later divide, as may be seen in fig. 78, and begin at 
once to form two bands of several cells each, which lie in the antero- 
lateral region of the gastrula and later in the anterior head region of the 
larva. 
Since the discovery by Lillie in 1895 of mesoderm which arose from 
the ectoderm in the Lamellibranch Unio, various other cell-lineage 
workers have arrived at similar conclusions concerning other forms. 
As is well known, Lillie found that the larval musculature of the Glo- 
chidium arose from a cell of the second quartet, 2a, which in cleavage 
gives rise to a cell toward the segmentation cavity, the descendants of 
which are mesodermal in fate. Conklin’s results, published in 1897, 
gave evidence that in the Gasteropod Crepidula ectodermal mesoderm 
arose in three quadrants, in this case also from the second quartet (2a, 
2b and 2c), but appearing much later then the “larval mesoblast”’ of 
Lillie, so late, in fact, that the exact cell origin could not be traced. 
