42 LIBBIE H. HYMAN. 



the anterior end. After they have disintegrated, disintegration 

 begins at the tip of the forebrain and proceeds down the axis, 

 meeting the disintegration progressing forwards from the pos- 

 terior end in about the region of the hindbrain. An embryo of 

 this stage, at the first appearance of the optic bulbs, is represented 

 in Fig. 35. The first drawing in this figure shows the normal 

 embryo, the germ ring not yet closed and somites not yet formed ; 

 the other three drawings give stages in the disintegration. Both 

 neural tube and mesoderm are involved in the disintegration 

 although naturally the fate of the mesoderm is less readily 

 ascertained, owing to its loose structure. 



In later stages of Fundulus embryos the disintegration gradi- 

 ents remain the same except that the secondary region of high 

 susceptibility at the posterior end gradually decreases in import- 

 ance. By the time that a number of somites have formed the 

 eyes are the most susceptible region of the embryo. Following 

 the eyes, the forebrain disintegrates and disintegration proceeds 

 posteriorly. The posterior end then begins to disintegrate and 

 disintegration proceeds slightly forwards from this region, meet- 

 ing the other wave of disintegration near the posterior end of the 

 embryo. The disintegration of an embryo of this age is shown 

 in Fig. 36. The first drawing shows the normal embryo, the other 

 four the course of the disintegration. The shrinkage of the 

 embryo accompanied by a sinuous bending of the neural tube, 

 which always occurs in later embryos in the killing solution, is also 

 illustrated. The fate of the somites could not be observed with 

 certainty and hence they are omitted but the high susceptibility of 

 the segmental plate region is shown in the second drawing of 

 Fig. 36. In still later stages of Fundulus embryos, the eyes are 

 no longer more susceptible than the forebrain but both disinte- 

 grate about simultaneously. The fate of the somites could not 

 be observed with certainty as they do not disintegrate readily. In 

 earlier stages the somites appear to disintegrate from the posterior 

 end forwards ; in later stages when a number of somites have 

 appeared the anterior and posterior somites seemed to be the most 

 susceptible, the middle ones less susceptible. 



The latest stages of Fundulus which were investigated are de- 



