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forwards. The anterior margin of the head is seen to be divided 

 into two parts, by a central eminence, which is shown from above, 

 in fig. 4, and from below, in fig. 3, on each side of this central 

 elevation are the three anterior metameres referred to. If we follow 

 the history of those metameres, we shall see that they shift their 

 position with reference to the other parts of the head, and attention 

 will be called to their changed position in some of the later stages. 



Following, in fig. 3, the headed edge from the head into the 

 trunk region we find it disappearing from view beneath the expanded 

 walls of the gastrular cavity. Viewing the same embryo from above, 

 fig. 4, the metameric segmentation is seen to extend the entire length 

 of the embryo, and, posteriorly, into its expanded parts. There is 

 no marked difference in the width of the segments of the anterior, or 

 the posterior parts of the body, they are approximately the same 

 size throughout the embryonic region. We may see that the anterior 

 part of the embryo, is clearly marked off from the posterior part, by 

 an abrupt downward bend (/") in the medullary folds, and, just in 

 front of this bend, the head end of the embryo is much expanded. 

 There are eleven metameres on the la- 

 teral margin of this cephalic plate. The 

 downward bend referred to lies just in 

 front of the point of future origin of the 

 vagus nerve. 



Fig. 5, represents an older stage, 

 in which the medullary folds have un- 

 rolled from their ventrally curved posi- 

 tion and are in the process of growing 

 upwards. They have at this stage risen 

 a little above the horizontal plane. The 

 outer margins of the head-folds are 

 plainly divided into segments, and the 

 segmentation extends backwards, also, 

 into the trunk region but not so clearly 

 defined. The optic vesicles (o p) are 

 clearly seen on the head-plate, but, the 



■ op 





Fig. 5. Older embryo after the medullary folds have begun to grow upwards. The 

 head-folds are now just above the horizontal plane. Reference marks as in the preceding Fig. 



accessory optic vesicles, have not yet made their appearance- 

 The head-plate is very broadly expanded and easily to be distinguished 

 from the more narrow part or trunk region. The line A A' separates 



