66 DEVELOPMENT OF THE TADPOLE. 



the same time the division of portions of the yelk mass into 

 segments or somites, together with decided indications of the for- 

 mation of the branchial arches, could be traced sections of the 

 animal at this period, as given in Plate VII. 



Fig. I shows the general external appearance of the embryo at 

 this date, whilst Figs. 2 and 3 give vertical longitudinal sections, 

 and Figs. 4 and 5 oblique ones, passing through the frontal nasal 

 process from between the letters g.—fn., Fig. 2, upwards through 

 the neural and body cavities. We thus obtain sections of some of 

 the more important organs which are now forming. Especially, 

 we notice the brain, the notochord, and pro-renal ducts. These 

 pro-renal ducts will be much better made out in the longitudinal 

 sections taken on March 8th and following dates. The frontal 

 nasal process is a ridge or protuberance extending the whole 

 length of the body and terminating at the top of the head, not 

 abruptly, but gradually merging into the rest of the epidermis. 

 As the animal grows, it is gradually obliterated. 



In Fig. I of this Plate attention should be drawn to the dark 

 mass on the lower portion of the head, which will be found in 

 future sections to consist of a muscular growth called the claspers, 

 whereby the animal, during a certain portion of its existence, 

 attaches itself to any neighbouring objects. 



In Fig. 2, Plate VII., the dark portions marked s. indicate 

 the position of the somites, segments which ultimately enclose the 

 vertebral column, so that the indentation in this and the next 

 figure really represents the back or dorsal region, whilst the oppo- 

 site side, which has more the appearance of a back, is actually the 

 ventral portion. It is thus seen that the greater number of 

 organic changes occur in the head and dorsal regions, whilst the 

 ventral side is still filled with undifferentiated yelk mass, merely 

 surrounded by the three layers of epiblast, mesoblast, and 

 hypoblast. 



Sections taken on the succeeding day (March 9th) showed an 

 immense advance in growth. All those organs which previously 

 were only indicated in jiosition by slight differentiation of the yelk 

 mass had now become more distinct, whilst other parts were 

 readily made out. 



Fig. I, Plate VIII., shows a vertical, longitudinal section of this 



