426 TREADWELL. [Vol. XVII. 



cases is the smaller and may be neglected in the further de- 

 scription. The inner products show from the first a tendency 

 to invaginate (see PI. XXXVIII, Fig. 28), where their nuclei 

 lie beneath the surface. Later each cell divides dorsally a 

 small cell, coming to the surface to divide (PI. XXXVIII, 

 Figs. 35, 3d2.i and 3d2.2, and 36, 3C2.1.2 and 3d2.2.2). Their 

 nuclei immediately start to invaginate again. Finally each 

 nucleus comes to the surface and a very small cell is budded 

 off (PI. XL, Figs. 50 and 51), and all four of the cells thus 

 formed invaginate to give rise to a large part of the larval 

 mesoblast, which is entirely the functional mesoblast of the larva. 

 As we shall see immediately, the larval mesoblast is completed 

 by the addition of other cells from another quadrant. 



Of the other products of 3d and 3c little need be said ; 3di 

 and 3c I divide meridionally soon after the first division of 3d2 

 and 3C2 (PI. XXXVIII, Fig. 28, 3di), and a number of later 

 divisions have been noted. (See PI. XXXIX, Figs. 40, 42, 

 44, 46, and table, p. 438.) They form a portion of the ventral 

 ectoderm of the trochophore, take part in the closure of the 

 blastopore, and make up a part of the proctodaeal wall. (See 

 PI. XXXIX, Fig. 46.) 



The first division of the third group of micromeres in the A 

 and B quadrants is shown in PI. XXXVIII, Fig. 28, where 3a2 

 and 3b2 are dividing meridionally. A little later 3ai and 3bi 

 divide also meridionally (PI. XXXVIII, Figs. 34 and 35). The 

 next division in A quadrant is of cell 3a2.2 (PI. XXXIX, Fig. 

 41). This divides unequally, sending a small cell upward, 

 3a2.2.i (PI. XL, Fig. 50), while the larger lower portion begins 

 to invaginate (PI. XL, Figs. 50 and 51). This cell, 3a2.2.2, 

 passes into the segmentation cavity and becomes larval meso- 

 blast. Originally situated at the left of the median plane, it 

 passes forward and lies directly in this plane. Its next division is 

 equal. (See PI. XL, Fig. 52, and Text-Fig. 7.) From this time 

 on its products lie symmetrically on either side of the median 

 plane. A corresponding cell in B quadrant divides at about 

 the same time as this and apparently invaginates (PI. XL, 

 Figs. 49 and 51). I at first supposed that this also becomes 

 larval mesoblast. Careful examination, however, fails to show 



