METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 5 



begin at the anterior end of the streak and progress posteriorly, 

 rapidly at first, then more slowly. About seven embryos of this 

 stage were observed and the course of disintegration was identical 

 in all of them. 



4. Stage of the Head Process. This stage is illustrated in Lillie, 

 Fig. 44 B, p. 87; its disintegration is shown in my Figs. 4 to 6. 

 Disintegration begins at the anterior tip of the head process 

 and proceeds posteriorly, at first very rapidly, indicating that 

 there is little difference metabolically along the length of the 

 head process, then more slowly. But one blastoderm of this 

 stage was observed although several attempts were made to 

 secure more of them. It seems that this stage endures but a 

 very short time. 



5. Stage of the Medullary Plate and the Head Fold. This stage, 

 illustrated in Figs. 7 to 10, about corresponds to Lillie's Fig. 44 C, 

 p. 87. Disintegration begins at the anterior end of the primitive 

 streak, at a point which is probably the primitive knot, and 

 progresses posteriorly along the primitive streak as shown in 

 Figs. 7 to 9. Disintegration is then initiated at the anterior end 

 of the medullary plate and proceeds posteriorly as shown in 

 Figs. 9 and 10. 



This stage marks the appearance of the "double gradient" 

 which has been shown to be characteristic of segmented animals 

 (Hyman, '16, '21 ; Bellamy, '19). There are two regions of high 

 susceptibility, the anterior end of the primitive streak and the 

 anterior end of the neural axis. 



6. Early Neural Fold Stage. The disintegration at this time is 

 similar to the preceding and is illustrated in Figs, u to 13. 



7. Appearance of the First Somites. An embryo with one 

 somite and two intersomitic furrows is illustrated in Figs. 14 to 

 17. It is similar to Lillie's Fig. 50, p. 96. Disintegration begins 

 as before at what I shall call the anterior end of the primitive 

 streak, i.e., the place where the primitive streak is differentiating 

 into notochord and somites. This place presents a club-shaped 

 appearance. From this point disintegration proceeds in both 

 directions, forward involving the somitic region, backward along 

 the streak to its posterior end. Soon the anterior end of the 

 neural folds begins to disintegrate and this progresses posteriorly 



