46 bulletin: museum of comparative zoology. 



Stage VII. 



The larvae of this stage measure from 10 to 10.5 mm. in length. The 

 duct, the median angle of the lateral mesoderm, and the mesonephric blas- 

 tema have migrated still farther mediad, so that the blastema now lies 

 at the ventro-median angle of the epimer. The most noteworthy change 

 is the transformation of the thickenings of the blastema into more clearly- 

 cut masses, which from this stage on I shall call blastulae (the '' Bla- 

 schen " of German authors). These, although still elongated antero- 

 posteriorly, are shorter than in the preceding stage and more rounded in 

 cross-section. Their nuclei have begun to place themselves radially 

 about the long axis of the blastula, leaving a clear protoplasmic area in 

 the centre. The first indication of this arrangement is to be seen in 

 Plate 2, Figure \1^ fnd.ms^nph. As will be seen in columns (7 and D 

 of Diagram 1 (page bQ), the blastulae are distributed as in Stage YI 

 (column B, Diagram 1), one of the first order and one of the second 

 order for each segment. As before, this statement is true only for the 

 more anterior somites. From somite 13 or 14 caudad those of the sec- 

 ond order become more numerous. Those lying in the regions where 

 the germ-cell mass exists are in close contact with it. The fact that 

 those anterior to it are invariably connected with the lateral plates 

 makes it doubtful whether the separation mentioned in the preceding 

 stage was artificial, or the continuity in the present stage is due to a 

 secondary fusion. 



Posterior to somite 15 the blastema becomes more slender and more 

 uniform in diameter. Although there are occasional swellings in it, 

 they are ill defined and irregularly placed, and may possibly not 

 represent blastulae. 



Stage VIIL 



The larva on which I base my descriptions of Stage VIII measures 

 12 mm. in length. Figure 19 (Plate 2) represents a portion of a cross- 

 section through the posterior end of somite 11 of both right and left 

 sides, and cuts a pair of blastulae near their centres. 



The epimers, as the result of the great increase in their dorso-ventral 

 diameter, now project far below the chorda, and those of the opposite 

 sides of the body have approached each other so that they now compress 

 the aorta slightly. In this stage the body cavity {coel.) is represented 

 by an actual lumen immediately ventral and lateral to the germ-cell 

 mass, the lumina of the opposite sides of the body being separated by a 

 thin layer of tissue, the mesentery. 



