403 



The rudiments of several organs have now appeared upon the 

 head; the most anterior of these organs is the primary optic vesicle 

 (op), just back of this, near the margin of the head-folds, is seen 

 the first accessory optic vesicle (A. op 1 ), and still further back, in 

 the same line, is the second accessory optic vesicle (A. op 2 ). Behind 

 the latter structure, the margin of the medullary fold is bent abruptly 

 downwards; this is a normal condition at this stage, and, of course^ 

 it is found in earlier stages. Behind the primary optic vesicle and 

 somewhat between it and the first accessory optic vesicle, is seen a 

 rounded eminence (He), which is the external indication of the man- 

 dibular cavity, and behind this, is the branchial pouch from which 

 gill clefts are subsequently formed. 



Directing our attention to the margin of the right medullary 

 fold, we note that it is clearly segmented through the head region, 

 and, backwards, into the trunk region, to the point where, in the 

 fig., it disappears behind the yolk. The metameres extend, in reality, 

 to the posterior limit of the body. The embryo is placed in such a 

 position that we also catch a view of the three anterior segments on 

 the left medullary fold. It will be interesting now, to note the 

 position of these three anterior neural segments, on the right 

 medullary fold, since they are best seen on that side. They have 

 evidently shifted in position with reference to the rest of the head. 

 In an earlier stage, they were all on the anterior margin directed 

 forward, but now they lie in front of the eye. Of course, this shift- 

 ing in position is brought about by changes in the medullary folds. 

 The fourth metamere is (at this stage) between the primary and the 

 first accessory optic vesicles, the fifth, and a part of the sixth, lie in 

 front of the latter structure. The sixth, is also located where the 

 margin of the medullary fold begins to bent downwards. The follow- 

 ing five segments (7th to 11th) occupy the reflected part of the 

 neural fold. The eleventh, as has been indicated, lies in front of the 

 future origin of the vagus nerve. 



The next stage to be considered (fig. 7) is after the complete 

 closure of the medullary groove. We possess now a particular ad- 

 vantage in tracing these segments, because, characteristic landmarks 

 belonging to the head region have become established , and , these 

 enable us to say with definiteness, what are the relations of the 

 segments to the rest of the head. The auditory vesicle has not yet 

 appeared; when first established, its centre occupies the space of the 

 segment marked 10. Sometimes, .in its earliest stages, the circular 

 area spreads over the space of the three segments marked 9, 10 and 



