402 



the two. Id the third part of this article, I shall refer again to this 

 morphological division of the embryo, and show its bearing upon 

 determining the limits of the head. There are in this figure, as in 

 the previous ones, eleven pairs of segments in the broadly expanded 

 region. There has been a slight change in position of the foremost 

 segments with reference to the rest of the head-plate. The three 

 anterior ones, are no longer upon a part of the margin that looks 

 forwards, but they have been shifted backwards, and that part of the 

 margin that was anterior, now, constitutes a part of the lateral 

 border. 



It should be borne in mind, that all the stages so far described 

 are very young; the earliest ones are before the formation of the 

 medullary folds, and the oldest one is just when the medullary folds 

 are arching upwards to form, for the first time, a medullary groove. 

 The mesenchymic somites have, in the interim, appeared in the trunk 

 region, and have produced faint surface indications in the median 

 parts of the medullary plate. 



Taking a considerable step forwards in the history of these 

 segments, we come to the condition represented in fig. 6. This 



■■■#** 



•3 



Fig. 6. Embryo with open neural groove, op primary optic vesicle A. op 1 , A. op 2 

 first and second accessory optic vesicles. He beginning of mandibular cavity. B Anlage 

 of branchiae. 1, 2, 3, first three neural segments in front of optic vesicle. 4 to 11, remain- 

 ing segments in front of point of origin of vagus nerve. 



figure shows a stage in which the medullary folds have attained a 

 nearly vertical position, they are about to bend towards each other 

 and meet in the median plane, but, as yet, they have not become 

 approximated in any part of their course, and, therefore, we have 

 an open neural groove extending the whole length of the embryo. In 

 this figure, the embryo is viewed obliquely from the right side. 



