380 JOHN BEARD, 



time and trouble to have made sections of each and every embryo to 

 the tail-end, quite apart from the prolonged study which such sections 

 would have entailed. An advanced skate embryo is made up of much 

 more tail than body. Sufficient sections were made to establish a few, 

 and perhaps the most important, facts about this later portion of the 

 system. When they are developed, and this is recognisable in embryos 

 of 20 mm and upwards, the cells posterior to the 31*t somite form a 

 series of sparsely scattered cells reaching to the end of the tail. 

 Thus the system is continued backwards from the 31-*^ somite, but in 

 a feeble fashion, and horizontal sections of the posterior portion of 

 the trunk and of the tail would reveal no mosaic arrangement, such 

 as that in Fig. 90, or in Fig. 92. The cells of the posterior region 

 only occur at intervals in Raja, sometimes two in a section, more 

 often only one, while in many consecutive sections not a single cell 

 may be encountered. Two figures of such cells in the tail have been 

 drawn on plate 26, Figs. 77, 77 a. 



In this posterior region some few of the ganglion-cells of the 

 mesoderm, to be presently described, also occur. Further, the nerve- 

 processes of these posterior cells are but feebly represented, and no 

 nerves like those to be presently described of another region have 

 been met with. The cells here also begin to degenerate pretty early. 



But, as already stated, no detailed investigations into the posterior 

 part of the transient system have been made; the reason for this 

 resting more in a recognition of the fact, if the truth were told, that 

 little or nothing of fundamental importance was to be gained by 

 further research. Moreover, there is a large and interesting material 

 of ScyUium, Mustelus etc. awaiting description. 



b) Peripheral ganglion-cells of the transient system. 

 Under this title, used purely in a descriptive sense, may be classed 

 all those ganglion-cells of the transient apparatus which do not lie 

 at the top of the spinal cord, but which are met with either in the 

 mesoderm, or in the myotomes, or just underneath the epiblast. They 



in the head-region, for at the anterior end there are practically no new 

 parts formed. It is true that there is growth forwards but only of 

 structures which had previously developed. It will be obvious from 

 the above that I am unable to agree with Paut. Mayer and van Wijhe 

 in the opinion that the blastoderm is part of the embryo. According 

 to my view it is to be regarded as a degenerate larva on which the 

 embryo takes its origin. 



