EMBRYOIOGY OF CLEPSINE. 291 



formed are numerous and scattered over the whole surface of 

 the blastomeres at the close of the neurulation, and later appear 

 as entoderm-cells. 



11. Owing to the forward pressure of the germ-bands^ z pair 

 of depressions (pharyngeal clefts) form at their junctions with the 

 cephalic mass^ which deepen into an invagination that finally 

 encircles the pharyngeal portion and forms \\\t pharyngeal atrium. 



12. Tiie mouth, ov ])hari/nfjeal orifice {p a), like the pharyn- 

 geal cavity, is an ectodermic invagination, which begins as a 

 slight depression in the centre of the cephalic area, at a point 

 corresponding nearly, if not exactly, with the centre of the four 

 original ectohlasts. 



The mouth invagination is at first continuous, by a linear 

 depression, with the primitive groove, which is formed by the 

 junction of the two germ -bands. 



13. l^YiQ co7)J2inction of the germ-hands, mh2kX\nowy with the 

 oblique forward direction of predominant growth, is accomplished 

 first at the cephalic ends, and from here progresses gradually 

 towards the anal end, which is reached about the time of 

 exclusion. The somatomeric segmentation follows closely upon 

 the union of the bands, and progresses in the same direction. 

 The final somatomere is completed at the end of one or two days 

 after exclusion, after which the remnants of the neuroblasts and 

 mesoblasts and the primary blastomeres {a, b, c) soon lose 

 their individuality, and form only a common mass of yolk, which 

 is driven back and forth by the contractions of the embryo. 



14. The ganglionic chain is formed from the eight roivs of cells 

 produced by the neuroblasts, and does not probably include a 

 other elements. The precise origin of the cerebral ganglia is 

 unknown. 



15. The number of pairs of ganglia corresponding to that of 

 the somatomeres is thirtij -three. Four of these are consolidated 

 in the sub(esophageal ganglia, eight in the ganglia of the disc, 

 awdi four in the terminal ganglia of the body. 



16. At the time of exclusion two rows of colossal cells (segment- 

 cells), products of the mcsoblast, are found on each side of the 

 median ventral line of the neurula beneath the neural elements. 

 The two median rows appear to be connected with the cells of 

 the segmental organs, but in what way is unknown. 



The two outer rows maintain nearly their original position in the 

 walls of the septa, but are finally lost sight of in the growing 

 tissues. The position of these cells and their prominence suggest 

 that they are the mother-cells of the male sexual organs. 



17. There are sixteen pairs of permanerd segmental organs, 

 fifteen of which correspond to thefifteen pairs of enteric diverticula, 

 aiid the sixteenth lies before the anterior pair of diverticula. 

 They arise from mesodermic cell-groups, of which two ate 



