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y\.tV- 



Fig. 4. Head of embryo of Squalus acanthias sometime after the closure of the 

 neural groove, the anterior accessory optic vesicle is shown at A. op 1 . 



lateral eyes, and we have here to decide the question not from ex- 

 ternal resemblances but from mode of origin. It should be remem- 

 bered that the accessory optic vesicles arise like the primary ones, 

 have the same histological structure and are present from the time 

 when the neural plate is broadly expanded. I have traced them step 

 by step from their early condition on the cephalic plate till they 

 enter the thalamencephalon. This is enough to settle the question 

 of their nature, nevertheless they constitute "the vesicles of the third 

 ventricle", but in character they are accessory optic vesicles homo- 

 logous with those which give rise to the lateral eyes. 



Comparing figures 4 and 5 we shall note that the mid-brain is 

 greatly increased in size — its growth has been relatively faster than 

 other parts of the head. In fig. 4 the diameter of the mid-brain is 

 about the same as that of the accessory optic vesicles while, in fig. 5, 

 the mid-brain is much larger. The accessory optic vesicles are seen 

 now to occupy the region of the thalamencephalon. A study of older 

 stages shows that the further transformation in this region involves the 

 continued enlargement of the accessory optic vesicles. The dorsal 

 wall of the mid-brain grows much more rapidly than the other parts 

 and in this way a marked cranial-flexure is produced with a bulging- 

 out in this region of the head. Although the accessory vesicles become 

 reduced they remain always distinct and may be followed with certainty 

 into the thalamencephalon. When the cups from the two sides come 

 together the pineal outgrowth arises. 



Fig. 5 shows also the neuromeres and other points of cranial 

 anatomy; the numbers affixed to the letter n refer to the neuromes 



