120 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



(Fig. 87). That this plate of mesoblast is on the side of the embryo 

 opposite that on which the blastopore was situated, is supported to 

 some extent by the facts above mentioned concerning the posterior 

 growth of the blastoderm. Further evidence of this is found in the 

 columnar shape of the cells, which is characteristic for those on the j 

 dorsal side \ moreover many embryos long retain a slight depression 

 marking the place of the blastopore, and the blastoderm (ectoblast) 

 cells in this region are often delayed in division in late stages, as well ' 

 as in the earlier stages, as may be seen when the position of the blasto- | 

 pore is definitely known. It should also be mentioned that the second I 

 polar cell, which lies dorsally (animal pole) in the yolk at the anterior 

 end, is often visible near the anterior extension of the mesoblast both 

 in sections and in transparent preparations of entire embryos corre- 

 sponding to Figures 87 and 88 (Plate 10). These facts all seem to 

 favor the conclusion that the forward growing band of mesoblast (Figs. 

 87, 88) is on the side opposite that occupied by the blastopore in 

 earlier stages, and consequently opposite that on which the mesoblast ex- 

 tends farthest forward at the time of the closing of the blastopore (Plate 

 8, Fig. 71 ; Plate 9, Fig. 80). 



Examination of Figures 88, 89 and 90 (Plate 10), representing long- 

 itudinal and transverse sections, will give some idea of the direction 

 and extent of growth in the mesoblast. A solid, conical mass of cells 

 lies at the extreme posterior end and extends anteriorly as a broad band 

 on the dorsal side (Fig. 88) ; this grows laterally towards the ventral 

 side (Fig. 90). The mesoblast at first consists of a single layer of cells, 

 which divide rapidly ; the layer becomes many cells in thickness on 

 the dorsal side, but gradually thinner towards the ventral edges of the 

 band (Figs. 90, 92). At the same time that the extension of the meso- 

 blast has been in progress, the entoblast cells have been dividing. 

 Their cell-boundaries are often well defined, and the nuclei do not 

 migrate far from the positions where they are formed by division (Figs. 

 91, 92). 



The blastoderm has remained a single cell in thickness, as shown in 

 the Figures 87-94. 



As shown in the preceding chapter, Groom's (94) view of the origin 

 of the mesoblast is erroneous, but the account which I have given of 

 the extension of the mesoblast is, in essentials, entirely confirmatory of 

 Groom's description of the same process. Groom has given many good 

 figures of entire eggs, showing the appearance of the entoblast yolk-cells 

 in living eggs of Lepas and Balanus. All my observations on these 



