352 EMBRYOLOGY. 



important proposition : the head- musculature is developed not only out 

 of the primitive segments, but also out of a part of the epithelium 

 of the head-cavity which corresponds to the lateral plates of the trunk ; 

 whereas the, latter do not contribute to the formation of muscles. 



So far as regards the dorsal part of the middle germ-layer in the 

 head-region, it is divided, as in the trunk, into primitive segments, 

 which in the Selachians are nine in number and embrace each a 

 cavity, with the exception of the first, which is solid. They arise 

 first in the posterior region of the head, and increase from there 

 forward. The segmentation of the whole body is therefore accomplished 

 in the Selachians and the same is likewise true for all the remaining 

 Vertebrates in such a manner that it begins in the neck-region, and 

 proceeds thence on the one hand backward to the tail, on the other 

 forward. 



The walls of the primitive segments of the head in part furnish 

 muscles, in part degenerate. Out of the first three pairs arise the 

 eye-muscles, as MARSHALL and WIJHE have demonstrated in detail. 

 The first segment envelops the primitive eye-vesicle like a cup, and 

 is differentiated into musculus rectus superior, rectus inferior, and 

 obliquus inferior. The second pair gives origin to the obliquus 

 superior, and the third pair to the rectus externus. The segments 

 from the fourth to the sixth inclusive disappear, while out of the 

 last three are developed muscles which extend from the skull to the 

 pectoral girdle. 



In the remaining Vertebrates the metamorphosis of the middle 

 germ-layer in the head has not been investigated in so exhaustive 

 a manner as in the case of the Selachians. There do not appear 

 to be any head-cavities developed, because the middle germ layers 

 remain at all .times pressed together. However, we know that 

 primitive segments are demonstrable even here. GOETTE describes 

 four pairs of them in Bombinator ; FRORIEP finds in Mammals in 

 the occipital region alone on either side four muscle-segments, of 

 which the two most anterior are believed subsequently to degenerate. 

 In individual cases there still remains much to be elucidated by 

 more exhaustive investigations. 



RABL has recently expressed dissent in some points from the 

 exposition of the head-segments as given by WIJHE. He divides the 

 head-segments into two groups four anterior or proximal, and five 

 posterior or distal. Only the latter are according to RABL to be 

 compared with the trunk-segments ; whereas the first, owing to their 

 method of origin, must take a separate position. 



