35S PROCEEDINGS OF THE ACADEMY OF [July, 



PL-4.TE XX^'. — Blastoderms, showing later stages of cleavage. 



Fig. 13. — Another S5-oell stage, eonsistiiig of 72 ectomeres, 6 mesentomeres 

 ajid 7 entomeres. 



Fig. 14.— Stage with 111 cells ; 98 ectomeres, 6 mesentomeres and 7 entomeres. 

 This is a more advaneed stage than the two following ones. 



Fig. 15.. — Stage with 89 cells; 70 ectomeres, 6 mesentomeres, 7 entomeres. 



Fig. 16. — Stage with 104 cells; 91 ectomeres, G mesentomeres, and 7 ento- 

 meres. 



Fi^"' 17. — Stage with 121 cells; 101 ectomeres, 10(?) mesentomeres and 7 

 entomeres. 



Fig IS. — Stage of about 320 cells. The ectomeres are very numerous and 

 are somewhat depressed below the general level in the region anterior 

 to the apical pole. The nuclei of the secondary macromeres, 4A, 45 

 and 4C, are dividing. 



Plate_XXVI. — Blastoderms, from the time of formation of the apical invagina- 

 tion to the appearance of the primordia of definitive organs. All 

 nuclei and cell boundaries, where shown, were drawii vnth 'the camera 

 lucida, so that their number and location are fairly accurate. 



Fig. 19. — Stage of approximately 512 cells. The apical invagination (A.I.) 

 is a deep pit just anterior to the animal pole. The mesentoblast (4c/) 

 has given rise to the mesodermal bands (Ms.). 



Fig. 20. — Stage slightly more advanced than the preceding, sho\\'ing the 

 apical invagination as a tubular ingrowth, with its opening near the 

 anterior edge of the blastoderm ; the nuclei of the secondary macromeres 

 (4A, 4B and 4C) are di\'iding a second time. 



Fig. 21. — Stage immediately after the flattening out of the apical invagina- 

 tion, only a trace of which is here sho-mi. On each side of the latter is 

 a grovip of cells, lying Ijeneath the surface of the blastoderm, and repre- 

 sented with solid black nuclei, whicli are probal^ly "larval mesoderm" 

 cells (L.M.) ; the superficial group of cells in this region ultimately gives 

 rise to the cerebral ganglion (Cb.), while the aggregation of cells lying 

 between and in front of the mesodermal bands (Ms.) is the primordium 

 of the shell gland. Two of the secondary macromeres (4.1 and 4C) 

 have, exceptionally, divided. 



J7g 22. — In this and the following figures all traces of the apical invagina- 

 tion have disappeared; in other respects this figure is similar to the 

 preceding one. 



Fig. 23. — Stage of about 1,000 cells, in wJiich the shell gland is plainly -visible 

 as a disk of cells, slightly depressed in the middle, and in which the 

 primordia of the cerebral (Cb.), buccal (Be), pleural (PL), and pedal 

 (Pd.) ganglia and of the intestine (In.) are recognizable as groups of 

 cells. The row of nuclei on the outer side of each cerebral ganglion 

 probably represents the primordium of the velum, while the more deeply 

 staining area connecting the two cerebral ganglia across the apical pole 

 probably corresponds to the "cephalic neural plate" of annelids. The 

 nuclei of the secondary macromere (iB) are dividing a third time, but 

 the cell body remains undivided. 



Fio-. 24. — Stage in all regards similar to the preceding, except that the 

 blastoderm has grown larger. The nuclei in the macromeres have here 

 reached their maximum size (cf. the nuclei of the same cells in figs. 7 

 and 10). 



Plate XXVII. — Entire eggs, showing stages from the appearance of organ 

 primordia to the completion of the overgrowiih of the yolk. 



Figs. 25 and 26. — Stages similar to the one sho^^^l in fig. 24. The primordia 

 of different organs are indicated by the closely stippled areas, the stipples 

 representing nuclei. 



Yig, 27. — Stage in \Yhich the blastoderm has extended through the growth 

 of its anterior portion, over about one-third of the yolk. The organ 

 primordia are confined to a small area in the posterior margin of the 

 blastoderm. The nutlei of the macromeres are dividing. 



