THE COMMON FROG. 



45 



lated appearance, or what is called a " mulberry mass." In the 

 frog's egg we have an example of unequal segmentation, although 

 symmetrical ; presenting the idea of a beautiful sphere, and 

 reminding us rather of our early lessons on the globe, than any- 

 thing we should expect to find in the egg of an animal. 



Fig. 16. — Eggs of frog, highly magnified. From Claus. 



Still more so when we hear of its " poles, axes, equator," etc. 

 One portion of the contents, lighter than" the rest, always turns 

 uppermost, producing an " upper " pole, the opposite being the 

 " lower pole " of the egg. The first cleavage (fig. 2) divides the 

 egg into halves, the next (fig. 3) at right angles, forms four divisions 

 and an "axis" between the poles. Then, as you perceive in 

 fig. 4, an "equatorial" division, but nearer the upper than the 

 lower pole, and is followed by longitudinal divisions, after which 

 the yolk soon breaks up into numerous smaller cells, as seen in 

 the above examples. 



One of the first indications of advance in the animal scale is 

 the presence of the noto-chord, the forerunner of the vertebral 

 column, or backbone, which is incipiently begun in the egg just 

 so soon as the segmentation process is completed. Then there 

 appears in the yolk what is called the " primitive groove, a delicate 

 longitudinal line, the sides of which rise up and arch over, forming 

 a canal, the lining of which becomes the spinal chord; while be- 

 neath it a gelatinous rod develops into a noto-chord {chorda dorsalis). 

 Hence ultimately the backbone. 



Now in the Tunicates, as Claus 

 explains to us, there is an embryonic 

 development, which up to a certain 

 point presents a great resemblance 

 to that of the lower vertebrates. Flg " ^- TBntate Tad P° le - 



First, the egg segmentation, subsequently a structure like a chorda 



