7 6 



CHILD. 



[VOL. I. 



The closure of the blastopore begins at the dorsal end. 

 The growth of the somatic plate (X) results finally in a con- 

 cresence of its two sides along the median line. The first two 

 cells to meet are derivatives of those called Xr, XI, in Fig. 10. 

 Just dorsal to their point of meeting lies the proctodaeal 

 region (Proct Fig. 10). This, though now filled with small 

 cells, I believe must be regarded as in reality a part of the 

 blastopore. The ventral portion of the blastopore, the portion 

 still open in Fig. 10, later forms the stomodaeum. The dis- 

 tance between the two is continually increased by the ventrally 

 advancing concrescence of the somatic plate. The cells of the 



-7-x 



FIG. 9. 



FIG. 10. 



blastopore lip are pushed together from behind and from each 

 side, principally the latter, until closure is effected. The 

 somatic plate continues to grow posteriorly, carrying at its tip 

 the proctodaeal region and the cells surrounding it, and thus 

 the line of concrescence between the stomodaeum and procto- 

 daeum increases in length, and growth in length of the larva 

 begins. The area covered by the somatic plate in different 

 stages is shown from the side in Figs. 8, 9, and 12, X. 



The center of the blastopore from the earliest stages is ven- 

 tral to the posterior pole, consequently the gastrula is not 

 radially but bilaterally symmetrical, and its main axis does not 

 correspond exactly with any of the future axes. These facts 

 are easily deduced from Figs. 9 and 1 1 . 



About as soon as the two mesoblasts are completely inclosed, 

 they begin the formation of the mesoblast bands. The spindle 

 for the first division lies almost dorso-ventrally, the smaller 



