METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 37 



filtered to remove the abundant precipitate which is always 

 formed on the addition of alkali to sea-water. 



The drawings illustrating the disintegration of fish embryos 

 were copied from free-hand sketches made while the embryos 

 were under observation. They do not pretend to be accurate 

 drawings of the embryos but are simple diagrams. The process 

 of disintegration is represented by stippling. 



2. Disintegration of Early Blastoderm Stages. The very early 

 cleavage stages were not investigated. The study began at the 

 stage -when a small blastoderm is present consisting of a number 

 of cells. At this time in both Tautogolabrus and Fundulus, the 

 central cells are found to be the most susceptible and from them 

 disintegration extends to the periphery of the blastoderm. Figs, 

 i and 2 show the course of disintegration in the egg of Tauto- 

 golabrus in an early blastoderm stage consisting of about thirty 

 cells. The most central cells become crenated at their margins 

 and eventually resolve into droplets ; this process spreads to the 

 periphery. Similar observations were made upon Fundulus eggs 

 but they were not entirely satisfactory. In the ammonium hy- 

 droxide solutions the blastoderm of Funditlns invariably shrinks 

 and arches up on the yolk. After this change has occurred, the 

 central region of the blastoderm bursts open and subsequently 

 disintegrates ; later this disintegration extends to the periphery. 

 These changes are illustrated in Figs. 3 and 4. The alkaline 

 solution disturbs the normal tension existing within the blasto- 

 derm and between the blastoderm and the yolk ; the blastoderm is 

 loosened at its periphery and shrinks into a mass much smaller 

 than normal. This occurrence has rendered it difficult or im- 

 possible to determine the course of disintegration in the early 

 stages of Fundulus. It is a question whether such changes as 

 can be observed, such as those represented in Figs. 3 and 4, are 

 real expressions of differential susceptibility or whether they may 

 not be due to the alterations of tension just described. In view, 

 however, of the observations on Tautogolabrus, about which there 

 cannot be any question, it is reasonable to believe that in Fundu- 

 lus also the central region of the blastoderm is more susceptible 

 than the periphery. 



