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accessory optic vesicles are very prominent and look like a second 

 pair of eyes lying behind the true lateral eyes. But at this stage, which 

 may be characterized by the first appearance of the auditory vesicle, 

 there is no distinguishing mark between embryonic fore-brain and 

 mid-brain. 



There is no sure criterion for telling where the embryonic fore- 

 brain leaves off and the mid-brain begins. The mid-brain region is 

 now very small, but later on it grows rapidly and becomes relatively 

 the most prominent part of the brain. Before this growth of mid- 

 brain begins the anterior accessory vesicles occupy the apparent (but 

 not the morphological) mid-brain region. 



The accessory optic vesicle may, from certain marks that remain 

 fairly constant, be identified with certainty, both at this stage and in 

 later stages. There are six bar-like neuromeres, belonging to the 

 hind-brain, lying behind them on the walls of the neural groove. 

 After the groove is closed these structures show from outside. The 

 auditory vesicle is formed in such a position that three of these 

 structures lie between it and the accessory optic vesicle. Whether, 

 after the neural groove is closed, they are neuromeres or cranial 

 ganglia is not of importance in the present connection. The auditory 

 vesicle, in the process of sinking below the surface, is shifted back- 

 wards, and finally, there comes to be five of these elevations in front 

 of it (fig. 5 n 6 to n 10 ) and, also, in the meantime, mid-brain and 

 cerebellum have appeared. But all these transformations go on so 

 clearly that no confusion need arise if one has a suitable series of 

 embryos. 



Fig. 4 represents an embryo sometime after the closure of the 

 neural groove, and after the auditory vesicle (au) is fully formed, 

 the primary optic vesicle is indicated at op and the anterior accessory 

 one at A. op 1 . Just behind this lies the mid -brain (mb). The 

 posterior pair of optic vesicles are no longer distinguishable. This 

 figure shows a condition intermediate between the stage when the 

 mid-brain is practically indistinguishable and a stage when it is very 

 prominent. It will be noted that the cerebellum is just appearing 

 behind the mid-brain, and that there are four neuromeres (instead of 

 three as in an earlier stage) between it and the otocyst. 



In certain stages the mid-brain is the same size as the accessory 

 optic vesicle and from external view they look very much alike. It 

 is inevitable that we should entertain the suggestion that what I have 

 called "accesssory optic vesicles" may be brain vesicles. But, 

 the external resemblance holds good also for the vesicles of the true 



