236 



INTERNAL FACTORS 



IV. i 



predetermined by the arrangement of certain substances in the 

 egg, and segmentation exhibits the character of a ' Mosaic ' work, 

 segregating these specific materials into the rudiments of definite 

 organs. The causal connexion between these materials and the 

 organs to which they are beforehand assigned is demonstrated by 

 experiment. 



m'ch 



FIG. 145. Cynthia partita. Half and three-quarter embryos. (After 

 Conklin.) a, right half gastrula, dorsal view. The neural plate, chorda, 

 and mesoderm cells are present only on the right side and in 

 their normal position and numbers, b, left half of young tadpole, 

 dorsal view. The notochord is normal except for size and number of 

 cells : the muscle and mesenchyme cells are present only on one side : 

 the neural plate (n.p.) is abnormal in form but not in position, c, right 

 half of young tadpole, dorsal view: slightly younger than 1. m'ch., 

 mesenchyme, ms., muscle-cells. <l, left anterior three-quarter embryo, 

 dorsal view. The anterior half is entirely normal with anterior mesen- 

 chyme (m'ch.) on both sides. Posterior mesenchyme and muscle-cells upon 

 the left side only. 



