BASIC REPRODUCTIVE AM) HEREDITY I'lIESOMENA '> 1 



follows and the developmental stages already noted in the study 

 of the starfish are completed in reg;ular order, n>sulting in a 

 multicellular body with all the necessary organs. When the 

 eggs hatch, the young animals arc not in the form nordothry 

 possess the habits of the adult. They enter upon u true larval 

 stngr which may be viewed as one occurring between the egg aiul 

 adult stages for the completion of developiiient. During the 

 larval period the structure and functions of the body arc such as 

 conform to its afiuatic life, necessitating many organs such as 

 gills aiul a swimming tail not present in the adult which leads a 

 terrestrial life. At the end of the larval period the larval struc- 

 tures are lost and those of the adult are assumed. 



Due to the presence of a large quantity of yolk in the frog's 

 egg and for other reasons, the details of development vary from 

 those found to obtain in the starfish. When properly viewed, 

 however, they are found to be fundamentally alike. 



Stages illustrating the early development of the frog may l)e 

 studied from preserved material. 



B}'- pouring the liquid containing this material into a watch 

 glass or other container the several stages may be studied with a 

 hand lens or, better, with a dissecting microscope. Small 

 camcl's-hair brushes are convenient for moving the material 

 in the liquid. Care must be used not to destroy the eggs and 

 embryos. Report any mishap or lost stages, that these may be 

 replaced immediately. Along with the prepared material it will 

 facilitate study to employ enlarged models of the same stages. 



1. One-celled Stage. — The greater part of the frog's egg is 

 composctl of yolk, which is used for the nutrition of the develop- 

 ing embryo. That portion of the egg in which the yolk is more 

 abundant, the vegetal hemisphere, or pole, is marked externally 

 by its white color, while the opposite, in which there is relatively 

 less yolk, the animal pole, is colored with black pigment. Due 

 to the large amount of yolk which it contains the egg of the frog 

 is relatively nuich larger than that of the starfish but because of 

 pigment granules the nucleus, which lies in the dark pole cannot 

 be seen. 



2. Two-celled Stage.— Cleavage begins at the animal pole 

 as yolk retards this process. The depression gradually extends 

 in the form of a groove until it finally surrounds the egg. This 

 groove marks the outer boundary of a cleavage plane which 

 extends through the egg, dividing it into approximately two e(iual 



