358 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 m each of the two anterior cjuadrants, 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 (PI. XXIX, 

 figs. 68, 69). During this period the third cpiartet 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 ol^served is the origin of the 



Ecto-Mesoblast. 



As the cells 3a"\ "-, ''\ '" and 3b"\. "-, '-\ '-- continue to move 

 toward the blastopore, the cells which they are covering over 3a^", 

 3a"^ and 3b-", 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^^", 

 31^2212^ while the larger daughter cells continue to retreat beneath 

 the overgrowing ectoderm (fig. 74). These larger cells, 3a-"S 3a-2", 

 3b-"^ and 3b"", 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 Gio- 

 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. 



