The Development of Ischnochiton. 655 



Fig. 35. About the same stage as Fig. 34, showing first definite 

 indication of the archenteron. Macromeres have in vaginated and fourth 

 quartette cells follow. Second quartette of the posterior quadrant oc- 

 cupies the bottom of a furrow. Secondary stomatoblasts of quadrants 

 A, B and G colored yellow. Division of the tertiary stomatoblasts 

 (anterior, smallest) and of 3a^-^ ^, 3a^'-^-'-^ and Sh^ l-'-^, 3 b ^•^■^. 



Fig. 36. Same stage as Fig. 34 from posterior quadrant. Division 

 of posterior basal cross cells, / (? ^. l-*. A i. i. i , idi.'-^.l.l.l/J ^ i d 1.2. 1.1. 2.1^ 

 ld^-2-i.l-2.2 Cleavage of upper row of cells in third quartette (second 

 row in first stages of division). Id^-^-^ divides. 



Plate 34. 



Fig. 37. Arrangement of macromeres, fourth quartette and meso- 

 blast in stage represented by Figs. 34, 35, 36. Formation of fifth 

 quartette. Right side. 



Fig. 38. Same as Fig. 37. Posterior view. 



Fig. 39. Later stage in the invagination. Anterior tertiary and 

 secondary stomatoblasts have divided and in vaginated to a considerable 

 extent; the posterior tertiary stomatoblasts remain on the surface of 

 embrj'o to a greater degree. Posterior furrow along line of second 

 quartette. Right and left secondary stomatoblasts are dividing. Cell 

 2 a 2. 2.1.1 divides bilaterally, also 2 a 2. 2. 1.2, 



Fig. 40. Division of fourth quartette 4 a, 4 b, 4 c. Cleavage of 

 macromere D. Same embryo as one represented in Fig. 39. 



Fig. 41. Late stage in the development of the rosette series 

 (Annelid cross) and of the basal and median cells of the Molluscan 

 cross. The lower hemisphere is somewhat triangular owing to the 

 tendency of the anterior third quartette cells to invaginate (first stages 

 shown in Fig. 39). 



Fig. 42. 31 hour embryo, showing commencement of the blastopore 

 migration ; the bilateral character of the second quartette. Anterior 

 rosette cells sbown. First appearance of velar cilia. 



Fig. 4.3. Upper hemisphere showing late stage in the development 

 of the rosette series and arrangement of basal and median (cross) pro- 

 ducts. Same stage as Fig. 42. 



Fig. 44. Same stage as Figs. 42 and 43 from left side. Growth 

 zone occupies the second row of third quartette cells. Second quartette 

 similar to one shown in Fig. 42. 



Fig. 45. Same stage as three preceding figures, from posterior 

 side, showing position of first somatoblast, and regular arrangement of 

 first quartette cells (products of the posterior arm of the cross). 



Fig. 46. Embryo of about 36 hours, dorsal view, showing ])erfectly 

 regular arrangement of the third quartette cells; the position of the 

 first somatoblast ; and location of the growth zone, Gz. 



Fig. 47. Side view of same stage as Fig. 46 from left side showing 

 position of second quartette, post-trochal cells, and regular arrangement 

 of the cells in the region of the future shell. 



