268 JOHNS HOPKINS QNIVEESITY MORPHOLOGICAL MONOGRAPHS. 



with included blastomeres (9). (The reference line which runs from the 

 (9) to the epithelium of the left gill tube is, as I have explained, the result 

 of, an erroneous interpretation.) Comparison of Fig. 5 with the sections 

 at higher levels will also show that this mass of cells hangs suspended in 

 the body cavity from the middle line of the dorsal surface, and that it is 

 pierced from side to side at the level of Fig. 2 by the atrium, g'". It is 

 divided by constrictions into four well-marked regions, and the later 

 history of the embryo shows that the lower one in the figure (s) is the 

 representative of the ganglion, and the second, which lies between the 

 gill tubes, that of the pharynx. I shall give, further on, my reasons for 

 regarding the third (19) in Fig. 4 as the representative of the notochord, 

 and the uppermost (18) in Fig. 4 as the representative of the nerve of the 

 tail. Comparison of Fig. 5 with Figs. 4, 2, 1 and Plate XVII, Fig. 5, will 

 also show that the blastomeres of the lowermost enlargement, s, of Fig. 5 

 and those of the uppermost, can be traced upwards until they become 

 continuous with the two ends of the string of ectodermal blastomeres 

 which lie outside the follicle on the middle line of the upper surface 

 covered only by the epithelial capsule. In the next section, Fig. 6, the 

 cavity of the placenta, y", almost completely surrounds the embryo and 

 its follicle, and the somatic follicle cells, 7, are in contact with the 

 supporting ring, 23, only at the two ends of the middle line. It will also 

 be noted that the blastomeres of the region of the pharynx extend out 

 into the planes which are occupied at a higher level by the perithoracic 

 tubes, Fig. 5, g. 



In the next section, Fig. 7, the cavity of the placenta, y", which is 

 colored yellow, almost, and in Fig. 8 quite surrounds the embryo, and it 

 will also be seen that the somatic and visceral layers of the follicle are 

 united on the middle line at the anterior end of the body. Sections 7, 8 

 and 9 are so simple that they will be intelligible without further explana- 

 tion when compared with cut B on p. 29. 



The most noteworthy features of the embryo at this stage are these : 

 1. The blastomeres show no trace of an arrangement in germ layers, 

 although they are divided into an extra-follicular ectodermal set, 9', and 

 an endodermal set, 9, imbedded in the visceral follicle cells, and form- 

 ing, with them, the embryonic mass. 2. The structure of the embryo is 

 vaguely outlined, but the outline is exhibited by the visceral follicle cells, 

 not by the blastomeres. 3. The cloaca and gill-tubes are more definitely 

 marked out, but they are entirely follicular, and consist of a thick outer 

 wall of visceral cells and an epithelial lining, derived from two paired 



