THE SKELETON (JF THE VEKTEBRATE HEAD. 8"J 



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

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

 atteutiou more particularly to the "dorsal" and "ventral 

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

 dorsalis," as fundamental embiyological characteristics of the 

 vertebrate type of organisation. 



Among the numerous results of the researches made by 

 Rathke in every department of embrj'ology, 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 vertebne, 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 luithke 

 on the development of the " branchial " or " ^'isceral laminie," 

 and of the nasal and maxillary portions of the face. 



Finally, Eumak has ascertained, on independent gi'ounds, 

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

 sidered as the rudiments of vertebras, becomes developed 

 superiorly into a right and left muscidar 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- 

 tebml system." The rudiments of the vertebral arch aud 

 ribs move backwards, from their original site under the 

 posterior margins of the overlying luuscular plates, to the 

 anterior margins of the pair of muscular plates immediately 

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

 sion takes place at the same time iu the nulimentaiy central 

 masses of each of the primordial vertebne. These changes 

 constitute a new order of parts — the order or aiTaugement of 



