V THE DEVELOPMENT OF THE FROG 83 



it was ascertained that the egg is a single cell. Not many 

 years afterward it was discovered that the spermatozoon is 

 likewise a single cell, and that in fertilization there is a union 

 of the nuclei of the ovum and spermatozoon, each contribut- 

 ing an equal share of chromatin to the nucleus of the fer- 

 tilized egg, and thence to all the cells of the body of the 

 embryo. The whole aspect of the problem of development 

 is very different from what it appeared to the older natural- 

 ists. However we may regard the modern forms of the 

 doctrines of evolution and epigenesis, — for both points of 

 view are still held, — it is certain that development actually 

 proceeds from a single cell to a body consisting of a multi- 

 tude of cells of great variety of form and function. This 

 cell may be enormously complex, containing somehow fea- 

 tures which represent all the different organs of the body, or 

 it may be comparatively simple in structure, and the differ- 

 entiations appearing in the embryo may be the results of the 

 interactions of its parts and the influence of external condi- 

 tions. Evolution and epigenesis both have their advocates, 

 but their differences have become less wide as knowledge of 

 embryology has advanced. 



The Jelly and its Uses. — The egg of the frog when it 

 is laid in the water is surrounded by a spherical mass of 

 transparent jelly. At first the coat of jelly is less than the 

 diameter of the egg in thickness, but through the absorption 

 of water it gradually swells until it becomes two or three 

 times this diameter. The jelly consists of three layers, a 

 thin inner layer closely applied to the egg, a thick middle 

 layer of more fluid consistency, and a thick outer layer. The 

 coats under the microscope show a concentric series of fine 

 lines, indicating the stratification of the material. The func- 

 tion of this jelly is primarily the protection of the eggs. It 

 keeps them free from dirt, bacteria, and the spores of fungi, 



