THE SKELETON OF THE VERTEBRATE HEAD. 89 



garding the so-called " serous " and " mucous layers/' took a 

 somewhat modified view of the " vascular layer," and directed 

 attention more particularly to the " dorsal " and " ventral 

 folds" of the blastoderma, in connection with the " corda 

 dorsalis," as fundamental embryological characteristics of the 

 vertebrate type of organisation. 



Among the numerous results of the researches made by 

 Kathke in every department of embryology, there are two 

 which bear particularly on the present subject. These are 

 his early discovery of the so-called branchial clefts ; and his 

 later recognition of the fact that the series of quadrilateral 

 bodies on each side of the " corda dorsalis," instead of being 

 the rudiments of vertebrae, contain potentially the germs not 

 only of these bones, but of the dorsal muscles, and "probably" 

 of spinal nerves. 



Eeichert supplemented the previous observations of Eathke 

 on the development of the " branchial " or " visceral laminse," 

 and of the nasal and maxillary portions of the face. 



Finally, Eemak has ascertained, on independent grounds, 

 that each pair of the dorsal quadrilateral bodies, usually con- 

 sidered as the rudiments of vertebree, becomes developed 

 superiorly into a right and left muscular plate, and inferiorly 

 into a pair of spinal nerves, with their ganglions, along with 

 the rudiments of a vertebra and pair of ribs, the nerves 

 being in front of the sclerous elements. In the course of 

 development a change takes place in this " primordial ver- 

 tebral system." The rudiments of the vertebral arch and 

 ribs move backwards, from their original site under the 

 posterior margins of the overlying muscular plates, to the 

 anterior margins of the pair of muscular plates immediately 

 behind, and become united to both pairs. A transverse divi- 

 sion takes place at the same time in the rudimentary central 

 masses of each of the primordial vertebrae. These changes 

 constitute a new order of parts the order or arrangement of 



