36 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



folded down on each side of them, so as to form a median dorsal ridge, 

 shown in cut B and in Plate XVII, Fig. 5. At first, as is shown in cut 

 A, these ectodermal blastomeres are not abruptly separated from those 

 at deeper levels, but as the perithoracic tubes and their derivatives are 

 formed and grow inwards, they separate the ectodermal blastomeres, in 

 the middle region of the body, from those of the visceral mass, as is 

 shown in cuts B and C. 



The series of sections on Plate XII shows, however, that there is no 

 such interruption before or behind the cloaca. At the posterior end of 

 the body, the end which is at the top in the figures, a continuous series of 

 blastomeres may be traced through all the sections, from 9' in Plate 

 XVII, Fig. 5, through 9' in Plate XII, Figs. 1 and 2 ; and 18 in Figs. 4 and 

 5 to 18 in Fig. 6. At the anterior end a similar series of blastomeres may 

 be traced from 9 in Plate XVII, Fig. 5, through 9' in Plate XII, Fig. 1 ; 

 9 in Fig. 2, and s in Figs. 4, 5 and 6, to Fig. 7. 



The mass of visceral follicle cells and blastomeres which makes up 

 the embryonic region is at first nearly spherical, as shown in the diagram 

 in Plate XII, Fig. 10. 



As the embryo grows and the body cavity becomes more capacious, 

 the visceral mass becomes folded into two vertical plates, intersecting 

 at right angles in such a way that a horizontal section shows it as 

 a cross, Plate XII, Fig. 5, with its long arm in the middle plane of the 

 embryo, and crossed near its anterior end by the short arm. In Fig. 1 

 the ends of the short arm are continuous around the spiracular openings 

 of the perithoracic tubes, with the somatic layer, as they are also in Fig. 

 2, where it contains the cloaea, which, with its lining of somatic cells, 

 runs across the middle line and perforates the long arm of the cross. In 

 Fig. 4, the enlarged rounded ends of the short arm contain the blind ends 

 of the perithoracic tubes, as also in Fig. 5, while in Fig. 6 and in Fig. 7 

 the short arm contains the blastomeres which are to form the pharynx 

 or branchial sac, into which, at a later stage, the perithoracic tubes open 

 through a single gill-slit on each side. The long arm of the cross is 

 formed by what is shown by the series of sections to be a thin verti- 

 cal plate of visceral follicle cells and blastomeres, hanging down into 

 the body cavity from an area on the middle line of the dorsal surface 

 where it is suspended from the layer of somatic follicle cells. In the 

 middle region of the body this vertical plate is interrupted by the peri- 

 thoracic organs, and it is perforated by the cloaca, but both below and 

 above the cloaca it is a continuous plate. 



