W. K. BROOKS ON THE GENUS SALPA. 37 



At the level of Figs. 5 and 6 which are in the planes indicated by 

 the lines 5 and 6 in cut B, the whole cross lies free in the body cavity. 

 At the level of Figs. 4 and 2 the ends of the short arm become continuous 

 with the somatic layer, and the long arm is interrupted by the cloaca. 



In Fig. 2 the posterior end of the long arm becomes continuous with 

 the somatic layer, as does its anterior end also in Fig. 1, where the long 

 arm is continuous above the cloaca, as it is below it in Fig. 4. 



Most of the organs of the body except the perithoracic system are 

 outlined in the visceral follicle cells of the median plate, and the blasto- 

 meres are grouped with reference to this outline. 



At the level of Fig. 4 there are three well marked enlargements of 

 the median plate, s, 19 and 18. One of these, s, lies anterior to the trans- 

 verse plate, and is the rudiment of the ganglion. In Fig. 2 it is about 

 as in Fig. 4, but in Fig. 1 its follicle cells become continuous with those 

 of the somatic layer, and in Plate XVII, Fig. 5, its blastomeres, s, become 

 continuous with the extra-follicular blastomeres of the ectodermal ridge 

 which is shown at A" in cut B. The enlargement of the median plate at 

 its posterior end, 18, is the rudimentary nervous system of the caudal 

 region, and in Figs. 2, 1, and XVII, Fig. 5, its blastomeres can be followed 

 up into the posterior end of the ectodermal ridge, just as those of the 

 ganglionic rudiment can be followed up into its anterior end. The third 

 thickening, which is marked 19 in Plate XII, Fig. 2, is the rudiment of 

 the notochord. At the level of Fig. 2 it contains no blastomeres ; at the 

 level of Fig. 3 it contains two blastomeres, but there are none in Fig. 4, 

 while they are numerous at lower levels, as is shown in Fig. 5 and also in 

 Fig. 6, where the notochord is erroneously marked 18 instead of 19. 



At the level of Figs. 6 and 7 the median plate, with its blastomeres, 

 swells out into a pair of lateral lobes which lie under the blind ends of 

 the perithoracic tubes, and give rise to the pharynx, as already described. 



All these structures are shown, in essentially the same relations, in 

 the older embryo in Plate XIII, except that the anterior end of the body 

 is at the top of the figures, and the caudal nervous system is marked 30 

 instead of 18, in both Plate XIII and Plate XIV. 



The most important changes in the visceral mass at this stage con- 

 cern the perithoracic system, and have already been described. 



The f ollicular rudiment of the caudal nervous system is also begin- 

 ning to break down and disappear. The degeneration begins at the 

 upper end, where it is continuous with the ectodermal ridge, and at this 

 stage, as shown in Plate XIII, Figs. 6 and 7, 30, it is now represented only 



