VH. GENERAL EMBRYOLOGY 



The egg cell, after the sperm cell has penetrated it (process of fertilization) 

 enters on a process of development, the study of which constitutes the subject 

 of embryology. An elementary study of embryonic development will be made 

 first on a simple case like that of the starfish (Asterias), then on the more com- 

 plicated case of the frog (Hegner, pp. 107-111). 



A. DEVELOPMENT OF THE STARFISH 



The fertilized egg first divides by mitosis a number of successive times until 

 a large number of very small cells is produced. This part of development is 

 called cleavage or segmentation. Study slide "Asterias early cleavage." Note 

 that the objects on the slide are not sections but the entire cells. (The cells 

 which exhibit a large clear nucleus containing a black spot [nucleolus] are unfer- 

 tilized eggs). Each egg and embryo is surrounded by a membrane, which is 

 called the fertilization membrane, and is separated from the egg at the time of 

 fertilization. 



1. Two-celled stage. The egg divides into two equal halves, which remain 

 in contact with each other. Find and draw in outline. 



2. Four-celled stage. A second division occurs at right angles but passing 

 through the same axis as the first. Draw. 



3. Eight-celled stage. Each cell of the four-celled stage divides in two 

 transversely at right angles to both the previous divisions, producing eight 

 equal cells in two plates of four cells each. Find and draw. Note that the 

 cells may easily become displaced from their natural position in the making 

 of the slide, and pick out only those that present the normal appearance. 



4. Later cleavage. Examine slide "Asterias late cleavage." The process 

 of division continues until a large number of cells is produced. Meantime, a 

 central cavity appears between the cells. 



5. Blastula. Examine slide "Asterias blastula or larval stages." At the 

 end of the cleavage process, the embryo consists of a single layer of cells sur- 

 rounding a central large cavity, the segmentation cavity. This stage is the 

 blastula. Its form is that of a rubber ball. Find one of these balls of small 

 cells, and focus so as to obtain an optical section, i.e., focus so that the appearance 

 is the same as if you had actually made a section through the center of the 

 blastula. In such a focus the blastula appears as a circular layer of cells, 

 the layer being one cell thick, surrounding a large cavity. Draw the optical 

 section. 



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