550 PATTERNS AND PROBLEMS OF DEVELOPMENT 



origin of mesoderm from cells of the second or of the third quartet is an 

 ancestral reminiscence, as has been suggested, it might be expected to 

 appear in greater degree in annelids than in mollusks, but apparently it 

 does not. 



The cell 2d is similar in size to other members of the second quartet, 

 and its descendants apparently give rise to no more ectoderm than the 

 other cells of the quartet in polyclads and nemerteans; but in annelids 

 it is usually larger than other members of the quartet, is called "first 

 somatoblast," and its descendants constitute most of the trunk ectoderm, 

 giving rise in oligochetes and leeches to the ectodermal teloblasts from 

 which the ectodermal germ bands develop (Figs. 172, Z), £, /^; 173, B-E), 

 but the cleavage patterns by which these results are accomplished differ 

 in almost every species studied. Among the mollusks the cell 2d may be 

 much larger than other cells of the second quartet, as in Unio (Fig. 174, 

 A), 01 the same size as others (Fig. 174, B, D). In Unio shell gland and 

 pedal ectoderm are derived from it (Lillie, 1895); and that this is also 

 true for Crepidula is held by Conklin (1897, iQo?)) i^"^ spite of the great 

 difference in size of the cell and time of appearance of shell gland as 

 compared with Unio. The third quartet (ja-jd) is regarded as ectodermal, 

 except that jb is said to give rise to entoderm in the oligochete Tubifex 

 (Penners, 1922). 



The cells 4a, 4b, and 4c of the fourth quartet become entoderm in cases 

 followed to this stage. In annelids and mollusks 4d, the second somato- 

 blast, is much larger than other cells of the quartet and becomes meso- 

 derm (M of Fig. 172, £, /^, and of Fig. ij^, B, D, E). In the leech. Clepsine, 

 according to Schleip (1914), 4D also becomes mesoderm (Fig. 173, D, E); 

 but in other forms it gives rise to entoderm, and what remains of the 

 "macromeres" after formation of the fourth quartet also becomes ento- 

 derm. 



In many annelid and mollusk eggs certain cytoplasmic regions are vis- 

 ibly distinguishable, either by absence of yolk or by presence of certain 

 granules. These regions may apparently undergo definite changes in posi- 

 tion in connection with maturation and fertihzation and be distributed 

 to particular cells during early cleavages. Very generally the amount of 

 yolk and other granular inclusions increases basipetally from the apical 

 polar region; but in many eggs a well-defined zone, aggregation, or ring 

 of cytoplasm containing little or no yolk is present about the apical pole, 

 either before maturation or appearing later; a second aggregation often 

 appears about the basal pole. These polar plasms are, or become, very 

 definitely localized in eggs of Tubifex (Penners, 1922) and Clepsine (Whit- 



