THE FROG. 29 



Fig. XX. Transverse section of Fig. IX., to show the mode of origin of the nervous 

 axis. D. 2. 



FIG. XXI. Transverse section, taken across the middle of Fig. XI. 



Owing to the tendency of the body-wall to shrink under the reaction of reagents, the 

 body-cavity, b.c., rarely appears as spacious as it is here figured. 



On the left side a slightly earlier stage in the development of the prorenal duct, re, is 

 represented, than would be seen at this period. See Furbringer (7), and Sedgwick (21). 

 D. 2. 



FIG. XXII. The anterior portion of a section, through the plane i. ii. of Fig. XI., 

 showing the origin of the visceral clefts and arches, and of the external gills. 



The right half of the figure indicates, in outline, the same parts at a later stage ; those 

 clefts which open up are thus represented, and the opercular fold and a third external gill 

 which had by this period come into existence, are both indicated by dotted lines. D. 2. 



The surfaces marked thus * are those which eventually bear the internal gills. 



FIG. XXIII. A section across the plane iii. iv. of Fig. XI., to show the mode of origin 

 of the essential parts of the eye, taken as a type of the higher sense organs. 



The right half of the figure illustrates the same at a later stage, after the lens has lost 

 all connection with the epiderma. D. 2. 



Note. Only the initial stages in the segmentation of the ovum are here figured, and the 

 central row of drawings, connected together by a longitudinal line, represents sections 

 through the same plane of a progressive series. 



Except in Fig. IX., the arrows follow the direction of growth of the cells at the point 

 indicated. 



al. Archenteron (midgut). 



a/'. Postanal gut. 



an. Aorta. 



a</. Aortic arches. 



mi. Auditory involution. 



li.c. Body cavity. 



/)'. Blastopore. 



In'. Branchial arches. 



lir.a. Branchial aperture. 



lir.c. Branchial chamber. 



lir.t. External gills. 



lif.f. Branchial fold (operculnm) 



hr.i. Internal gills. 



c.c. Cleavage cavity. 



cd. Caudal appendage (tail). 



fh. Notochord. 



ch'. Subnotochordal rod. 



c.v. Cerebral vesicle (brain). 



t>/>. Epiblast. 



/./. Fore limb. 



fr. Fronto-uasal process. 



h. Heart. 



hj. Horny jaw. 



h.l Hind limb. 



lip. Hepatic diverticulum. 



liy. Hyoid arch. 



/'. Coiled intestine. 



/, Crystalline lens. 



