40 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



disappear, they are possibly the spaces which they occupied. If this is 

 true, the rudiment of the true blastodermic chorda degenerates in Salpa 

 pinnata before its follicular case, but both structures soon break up, and 

 are used as food. 



We have traced the migration of the ectodermal blastomeres from 

 the interior of the visceral mass to their extra-follicular position in the 

 ectodermal ridge, Plate XVII, Fig. 5, 9, and cuts B and C, a". In this 

 position they are covered up by the epithelial capsule, but are on the out- 

 side of the embryo. At a stage a little older than Plate XIV and Plate 

 XXII, they begin to multiply and to spread out over the embryo on both 

 sides of the middle line to form the ectoderm. Plate XLII, Fig. 11, is a 

 transverse section of the ectodermal area of an embryo a little older than 

 Plate XIV. 21 is the outer fold of the embryo sac, 22 its inner folds, and 

 6' is the epithelial capsule ; a is the ectoderm spreading out at the sides 

 between the epithelial capsule and the somatic layer, 8, of follicle cells. 

 At this early stage in the development of the ectoderm its cells and nuclei 

 are so much larger than those of the follicle that they can be distin- 

 guished clearly, and the nuclei of the blastoderm cells are rich in chro- 

 matin and have a well marked reticulum with nucleoli and large granules, 

 and are in this quite different from the vesicular nuclei of the follicle 

 cells. The ectoderm has a growing edge, like that of a meroblastic 

 embryo, and it gradually spreads on all sides, and pushing under the 

 separated cells of the epithelial capsule, forces them off, and thus finally 

 becomes the outer covering of the embryo. The embryo, which is shown 

 in Plates XVI and XVII, is almost covered by the ectoderm, and Figs. 6 

 and 7 of this plate and 9 of Plate XLII are parts of this embryo, more 

 highly magnified to show the details of its structure. In all these figures 

 a is the ectoderm, and &' the epithelial capsule. The cellulose mantle 

 makes its appearance as soon as the ectoderm is fully formed, as a trans- 

 parent layer, which is shown at v in Plate XVII, Fig. 6, and in Plate 

 XLV, Figs. 3, 4 and 6. 



A few small cells are sometimes included in it, as in Plate XVII, 

 Figs. 5 and 6, v, but the cells of the epithelial capsule are on the outside of 

 it, as is shown at v in Plate XLV, Fig. 3. 



We have seen that the blastomeres of the ganglionic rudiment, s, are 

 at first continuous, above with the extra-follicular blastomeres of the 

 ectodermal ridge, as shown in Fig. 5 of Plate XVI, where s marks the 

 ganglionic blastomeres. At a lower level, Plate XII, Fig. 1, s, they are 

 imbedded in follicle cells at the point on the middle line of the body 



