32 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



scattered blastomeres, &Z, and is covered above and at the sides by a layer 

 of somatic follicle cells, 7. 



The degeneration of the follicular lining of the cloaca begins before 

 the gill-slits are formed. Fig. 7 of Plate XLII is part of Plate XIII, Fig. 

 8, showing part of a perithoracic tube at the stage when it ends blindly. 

 The visceral follicle cells are shown on the left ; then the follicular epithe- 

 lium of somatic cells, and then three of these cells, with double nuclei, 

 which have become amoeboid, and have wandered into the lumen of the 

 tube. Fig. 6 is a section through the axis of the gill-slit, from a specimen 

 at the same stage as Plate XXII, showing the follicle cells breaking apart 

 to form the channel of communication with the pharynx. 



As the embryo grows the pharynx increases in size, at first, very 

 much faster than the cloaca, and at the stage shown in Plates XVI and 

 XVII, the pharynx, Fig. 2, c, is very capacious, while the cloaca, XVI, 

 Figs. 1 and 2, g" and g lv , is very small. The relative sizes of the two 

 structures are also shown at a somewhat later stage in Plate XVIII, Figs. 

 4 and 8, c and g'", and also in the surface view in Plate XLI, Fig. 2, c and 

 f/". The two spiracular tubes seem to fuse into one by the disintegration 

 of the partition, as shown in Plate XVI, Fig. 1, g" ; and their chamber 

 becomes part of the cloaca. All the somatic follicle cells of the cloaca 

 ultimately fall into its cavity and degenerate, although this process is not 

 completed until the other organs of the body are well advanced in their 

 development, and they are shown in Figs. 1, 2 and 3 of Plate XVI, and at 

 x in Fig. 9 of Plate XLII. 



SECTION 5. The Blastodermic Tissties of the Embryo. 



We have now to consider the way in which the blastodermic epithe- 

 lium of the pharynx and cloaca replaces this temporary scaffolding of 

 follicle cells. 



While the changes which we have described are taking place, the 

 blastomeres gradually become smaller and more numerous, as shown in 

 Plate XLII, Figs. 1, 2, 3, 4, 5, and those which are to give rise to the 

 epithelium of the pharynx and cloaca become distributed through the 

 mass of visceral follicle cells under the region of the perithoracic tubes and 

 cloaca, as shown in Plate XII, Figs. 6 and 7, 9, and in Plate XIII, Fig. 9. 

 As the cavity of the pharynx is formed they become arranged between 

 the visceral follicle cells, Plate XLII, Fig. 8, 8, and the degenerating 

 somatic cells, 7, as is shown at bl in Fig. 8 and in Fig. 5. In this way the 



