INDIVIDUATION AND REPRODUCTION 



205 



Fig. 69, a longitudinal 

 section near the median 

 plane of the embryo of a 

 turbellarian worm, Plagio- 

 stomum girardi, shows very 

 clearly both the antero- 

 posterior and the ventro- 

 dorsal gradients. At this 

 stage only the head and 

 ventral region of the ani- 

 mal are represented by cell 

 masses, the regions where 

 the more dorsal structures 

 will later develop con- 

 sisting chiefly of yolk. 

 Moreover, the anterior re- 

 gion is more advanced in 

 development than any 

 other part. Fig. 70 is the 

 embryo of the earthworm. 

 In the anterior region the 

 body has attained its final 

 form, but posteriorly the 

 segmentation is more and 

 more limited to the ventral 

 region, the dorsal region 

 being little more than a 

 yolk sac, and in the ex- 

 treme posterior region seg- 

 ments have not yet become 

 visible. In the arthropods 

 the relations are in general 

 similar. ' The embryology 

 of other invertebrate 

 groups indicates more or 

 less clearly the existence 

 of symmetry gradients, but 



FIGS. 69, 70. Axial developmental gradients 

 in embryonic stages of invertebrates: Fig. 69, 

 a somewhat oblique, longitudinal (sagittal) 

 section of the embryo of a turbellarian worm, 

 Plagiostomum girardi; the cephalic ganglia and 

 eye at the left are advanced in development, 

 as is also the pharynx, but farther posteriorly 

 fewer cells are present; the ventral (lower) 

 region is also much farther advanced than the 

 dorsal (from Bresslau, '04); Fig. 70, advanced 

 embryo of the earthworm Lumbricus agricola: 

 development is more advanced anteriorly and 

 ventrally than posteriorly and dorsally (from 

 Kowalewsky, '71). 



