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embryo. The optic vesicles proper cover a space of three somites, 

 and, therefore, the whole system reaches back to the beginning of the 

 tenth embryonic somite. In Figs. 5 and 6, there are four accessory 

 vesicles extending backwards to approximately the eighth embryonic 

 somite. In Fig. 7, there are two accessory vesicles terminating upon 

 the front part of the fifth somite. 



Their entire history is comprised within a period of three or four 

 hours. They arise about the 24th hour of development — possibly a 

 very little earlier — their period of greatest differentiation is reached 

 about the 25th hour, after which they begin to decline, and their last 

 remnants may be seen about the 27th hour of incubation. But within 

 these limits there is considerable variation. 



The most essential thing in observing them is to possess good 

 clear specimens of precisely the right age. They are so transitory 

 that it is difficult to procure stages showing them at their highest 

 point of development. As every one knows, there is great variation 

 in the point of development reached by eggs that have been incubated 

 for the same period. Out of about 500 eggs, I was not able to se- 

 cure more than 50 specimens of the proper age to show this system 

 in any of its stages of development. In three cases, I observed the 

 accessory vesicles as shown in Fig. 2, while the embryos were still 

 living. Extreme care should be taken to wash the embryos clean by 

 a gentle jet of the reagent immediately, after immersion in it. The 

 embryos should be lightly stained, and studied by reflected light over 

 a dead black background. The manipulation of the embryo with re- 

 ference to the light is very important. All observers know that even 

 as large structures as the auditory vesicles in the chick, may be en- 

 tirely out of evidence, if the light is not right. One may look directly 

 down upon a specimen possessing auditory vesicles, without seeing 

 them, but, if the specimen be tilted or rotated, they show very clearly : 

 It is of course a question of getting shadows. It is, therefore, not to 

 be expected that the accessory eye - vesicles , which are much more 

 difficult to detect, can be seen without especial arrangement of the 

 light. I have found them showing best with the head -end turned 

 awayifrom the light, and slighth, elevated so as to be viewed obliquely. 



We will now, for a moment, look at two stages showing the brain 

 vesicles, after the accessory eye-vesicles have disappeared. 



Fig. 8, shows a stage about two hours older than that represented 

 in Fig. 7, and in which the brain vesicles proper have appeared. Their 

 formation is due to unequal growth and is accompanied by a widening 

 of the whole cephalic tube. The dorsal wall of the brain has expanded 



