174 Edwin G. Conklin. 



In Fig. 34 the left anterior quadrant was killed and the posterior 

 portion of this embryo is normal save only for the fact that the 

 notochord and nerve tube are smaller than usual, which is ex- 

 plained by the fact that the substance of these organs is derived 

 from the anterior quadrants; three rows of muscle cells are found 

 on both sides of the tail. The anterior half of this embryo, on the 

 other hand, is quite defective; the neural plate is irregularly folded 

 and has not formed a sense vesicle, although sense spots are present. 



I have seen and studied many three-quarter embryos sim- 

 ilar to those shown in Figs. 34 and 35 and they all show, as do 

 the right and left half embryos, that where part of the substance 

 which would normally form an organ is destroyed the organ which 

 develops is defective, whereas if all or any organ-forming substance 

 is lacking the organ to which it would normally give rise is also 

 lacking. 



So far as I have observed these partial larvre never escape from 

 the egg membrane, and in this my observations accord with those 

 of Chabry and Driesch, and although I have kept them alive until 

 a period after the normal larvie have undergone metamorphosis I 

 have never observed this transformation in them. 



In conclusion then I find that the cleavage and gastrulation of 

 these half or three-quarter embryos is partial and the resulting 

 larva incomplete although the notochord is well formed and there 

 is a tendency on the part of some of the cells to grow over and close 

 up the open side of the larva. However, this regulation never 

 leads to the formation of a complete larva; the neural plate ma^^ 

 close, but it forms an abnormal sense vesicle; at the end of the tail 

 the muscle cells extend over toward the injured side, but they do 

 not form three rows of cells on each side of the notochord as in the 

 normal larva; the mesenchyme likewise does not develop along the 

 injured side and it is probable that only one atrial invagination is 

 formed. 



Furthermore not a single cleavage cell nor any one of the ooplas- 

 mic substances ever gives rise to parts or organs which it would 

 not normally produce; the notochord, for example, invariably comes 

 from the chorda cells, the sense vesicle from the neural plate cells 

 and both these structures from the material of the gray crescent; the 

 muscles always come from the muscle cells and these from the sub- 

 stance of the yellow crescent; the ectoderm, frojn the ectoderm cells 

 and ultimately from the clear protoplasm; the endoderm, from the 



