ORIGIN OF MONSTERS 515 



In microscopic sections the embryo's right eye appears to be 

 perfectly normal in every respect, while the left eye appears in 

 the sections posterior to it and extending into the yolk-sac, is 

 diminutive in size, but well differentiated in essential structural 

 details. The optic nerve of the small eye is seen entering the 

 brain independently of the same nerve of the other (normal) eye. 



On examination of the entire series of sections the probable 

 nature of the injury sustained by the embryo is gradually dis- 

 closed. It is restricted mainly to one side and the abnormaUties 

 concern the olfactory pit, the eye, the brain, and the ear vesicle. 



While the olfactory pit of the uninjured side is normal and in 

 its typical position antero-median to the normal eye, there are 

 three olfactory pits on the abnormal side, all of which are closely 

 approximated (fig. 72). This points to a fragmeDtation of the 

 potential rhino-ectoderm at a pre-differential stage of develop- 

 ment. The condition of the eye of this side has been described; 

 its probable manner of formation is suggested by a careful exami- 

 nation of the brain. The latter in successive sections is found 

 to consist of two hemispheres, but ^s strikingly asymmetrical in 

 regard to the position occupied by them in relation to the chief 

 body axis, the right hemisphere preceding in all sections the left 

 one which is pushed posteriorwards (fig. 73). This agrees well 

 with the posteriorward dislocation of the eye on the same side 

 and is probably due to regulation after a sustained unilateral 

 lesion. Striking evidence of a process of dissociation (blastoly- 

 sis) is found on examination of sections at the level of the optic 

 lobe of this side (fig. 74). The latter, which, like the whole 

 hemisphere of this side, is posterior to the one of the right side, 

 is seen to be at the same level with the posterior part of the left 

 (smaller) eye, with the right ear, the heart, and the yolk of the 

 body cavity. It is incomplete, and looks as if a part of it had 

 been broken off. To the right from it (in the figure) there is a 

 wide cleft. Between it and the head integument there is a large 

 fragment of tissue (o. I. f.) which has the appearance of a small 

 optic cup at an early stage of differentiation, but on careful 

 examination of all sections is found at its most ventral point to 

 be in connection with the optic lobe. This tissue fragment must 



THE JOURNAL OF EXPERIMENTAL ZOOLOGy, VOL. 21? NO. 4 



