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JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



cular origin until the plate was finished. The orange color is used in the 

 same way for the follicular epithelium of the cloaca and perithoracic 

 tubes in Plate XXII. 



Below the level of the cloaca the perithoracic tubes run downwards, 

 one on each side of the middle line as shown at g 1 in Fig. 3 and at g v in the 

 cut, but they end blindly at this stage, and in Fig. 4 the left is cut at its 

 end below the cavity, while the right, gr, is cut close to the bottom of the 

 cavity. The next section, Fig. 6, does not cut any part of the perithoracic 

 tubes, although their positions are outlined on each side of the middle 

 line in visceral follicle cells with scattered blastomeres. The next stage 

 in the development of the perithoracic system is shown in Plate XIII 

 and in cut C. 



CUT C. 



A reconstruction in a vertical transverse plane from the horizontal 

 sections shown in Plate XIII. 



The cloaca, Fig. 7, g'", and the two prolongations of the tubes down- 

 wards, Fig. 8, g lv , are about as before, except that these latter are a little 

 longer. The spiracular tubes, however, have undergone a great change, 

 as they have lost their external openings and have moved towards the 

 middle line, where they lie side by side at g", in Fig. 6, above the cloaca, 

 imbedded in the visceral follicle cells. 



In Plate XIV and in cut D, the pharynx, c, is shown, as well as the 

 two gill-slits, gf lv , which have been formed out of the descending portions 

 of the perithoracic tubes, Fig. 6, g lv , which now open above into the cloaca, 

 Fig. 5, g'", and below into the pharynx, Figs. 8, c and 9, c. In Figs. 3 and 

 4, the two spiracular tubes g" are shown side by side, above the cloaca, 



