THE EARLY DEVELOPMENT OF THE FROG 79 



eggs (2-3 mm.), while the European water-frog (Rana escu- 

 lentd) in which the eggs are small (1.5 mm.) lays from five to 

 ten thousand during each season. 



III. EMBRYONIC PERIOD 



A. FROM FERTILIZATION THROUGH GASTRULATION 



1. Fertilization and the Development of the Symmetry of the Egg 



During the first hour or two after the entrance of the sperm 

 several changes of great importance occur within the egg. 

 Although these are going on together and overlap to a certain 

 extent we shall have to describe them separately. 



Entrance of the Spermatozoon. Within a few moments after 

 ensemination, a sperm cell bores its way through the thin jelly 

 and the chorion, and enters the egg substance; in most cases 

 the entire spermatozoon enters. Although there is no micro- 

 pyle the sperm does not enter the egg at random, but normally 

 only in the pigmented hemisphere, more frequently about forty 

 degrees from the animal pole, and in any meridian (Brachet). 

 The meridian passing through both poles of the egg and the 

 point of entrance of the sperm is known as the fertilization 

 meridian. 



Of course many sperm tend to enter the egg but the entrance 

 of the first seems to alter the chemical structure of the egg in 

 such a way that additional sperm are deterred from entering; 

 frequently many such sperm may be seen in the egg jelly. 

 Polyspermy, although not rare, is never normal in any of the 

 frogs, and should more than one sperm succeed in entering, 

 the development of the egg becomes abnormal. 



Once within the egg substance the sperm head and middle 

 piece move rapidly inward, following approximately a radius 

 of the egg. The path of the sperm is marked by a distinct 

 trail of pigment, indicating unusual metabolic activity of the 

 region, which remains visible for some time, occasionally even 

 to the blastula stage. The first part of the sperm path is 

 called the penetration path (Fig. 27, A, B). After the sperm 

 has travelled along this path for a short distance it rotates, 



