28 SUMMARY OF OUERENT RESEARCHES RELATING TO 



some property special to this hemisphere. If, however, an egg be 

 turned upside down during the process of division, it is found that 

 the cleavage proceeds more vigorously in what is now the upper half 

 and ceases to be so well marked in the lower half, and therefore clearly 

 has nothing whatever to do with any special quality of different 

 portions of the egg itself, but depends entirely upon its position. 



Another point to which Dr. Pfliiger directed his researches was 

 the relation that exists between the first cleavage and the axis 

 of the future embryo ; the experiments made appeared to show that 

 the two are identical, and that each of the two cells therefore formed 

 by the first division corresponds to one half of the body of the future 

 embryo : and also, a fact of the greatest importance, that the various 

 parts of the body appeared to arise from the light or dark hemispheres 

 according to the position of these latter ; when the light hemisphere 

 was uppermost the whole nervous invagination was seen to be clear 

 and transparent. This part of the subject, however, the author does 

 not consider to be as yet placed on a firm basis and intends to continue 

 his investigations. 



In a second paper on the same subject* the author notes that 

 among the tadpoles developed from the eggs some were remarkable 

 by the dorsal surface being entirely free of pigment, owing to the 

 fact that the light hemisphere had been fixed in the uppermost 

 position ; later, however, the pigment seemed to spread over the 

 whole body, and no recognizable difference between the dorsal and 

 ventral surfaces could be detected. The albinos also showed occa- 

 sional abnormalities and soon died. A further investigation was 

 made upon the eggs of Bomhinator igneus ; the main results appear to 

 be the following : Quite normal embryos were developed when the 

 upper hemisphere had a larger clear portion ; but if it became almost 

 entirely made up of the clear hemisphere the embryos were abnormal 

 and died, though it was perfectly evident that the axis of the egg 

 might be at any angle whatever with the direction of gravity, and not 

 interfere in the least with the early developmental stages. 



In the earlier communication it was stated that the axis of the 

 embryo coincided with the axis of the first cell-division, and that the 

 central nervous system was formed out of the dark or the clear hemi- 

 sphere, or out of both according to their position ; a more careful 

 investigation has shown that the central nervous system is always 

 developed from the clear hemisphere. 



This fact appears to show that the egg is after all not 

 " isotropous," and that a given organ arises from some particular 

 region of the egg entirely independently of gravity; but another 

 series of facts tends towards the opposite conclusion; in eggs fixed in 

 an abnormal position the anus of Eusconi was never to be seen upon 

 the upper hemisphere ; again, when the primary axis was inclined at 

 an angle the medullary groove was always developed with the anterior 

 end in the upper portion and the posterior end in the lower portion of 

 the white hemisphere, which latter, of course, was also obliquely 



* Pfliiger's Arch. f. gesammt. Physiol., xxxii, (1883) pp. 1-79 (2 pis.). 



