286 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



ping into new places, the soldiers grow until they are pushed out by 

 mutual pressure, and suppose that their heads, growing fastest, form 

 two rows while their feet still form one row, and suppose, furthermore, 

 that, as each soldier rotates, his feet turn first, and that the twisting runs 

 slowly up his body to his head which turns last. We must also imagine 

 that these various changes all go on together, and that while they are 

 taking place each soldier not only grows larger, but also develops from a 

 simple germ to his complete structure. 



Salensky regards the stolon as two rows of rudimentary salpae, and 

 while Seeliger correctly states that they first form a single row, he has 

 failed to discover the rotation, and believes that they arise on the stolon 

 in their final positions, and he has, therefore, failed as completely as 

 Salensky in his efforts to trace the origin of their organs. 



SECTION 3. The Ectoderm of tJie Salpa Embryo. 



I have already, page 40, described the origin of the ectoderm of the 

 salpa embryo. Briefly recapitulated, its history is as follows : 



At an early stage of segmentation, some of the blastomeres, Plate 

 XI, Fig. 1, 9' and Fig. 4, 9', move upwards and pass out of the follicle 

 on the middle line of the dorsal surface where the two layers are con- 

 tinuous with each other. I have given on page 63 my reasons for 

 believing that this is the spot which was once occupied by the blastopore. 



The ectodermal blastomeres, Plate XVII, Fig. 5, 9', thus become 

 extra-f ollicular, although they are covered for a time by the epithelial 

 capsule, &'. They give rise by cell division to the ectoderm, Plate XLII, 

 Fig. 11, a, which spreads from the dorsal middle line outwards and 

 downwards over the embryo, pushing off and replacing the cells of the 

 epithelial capsule, Plate XVII, Fig. 1, b' and Fig. 6, &'. 



The ectoderm has a growing edge, Plate XVIII, Figs. 4, 5 and 6, like 

 that of meroblastic embryos, page 57, and it does not close in completely 

 on the ventral middle line until after birth. 



Salensky has figured the migration of blastomeres to an extra- 

 follicular position on the dorsal surface of the embryo in several species, 

 although he seems to regard them as discarded blastomeres, and derives 

 the ectoderm from other sources. 



They are clearly shown in Salpa pinnata, in his Plate XII, Fig. 26 ; 

 in Salpa pectinata, in his Plate XXIII, and in Salpa fusiformis, in his 



