50 LABORATORY STUDIES IN ZOOLOGY 



5. The process of segmentation continues until a large number 

 of cells is produced. Some slides may show the 16- or 32-cell 

 stages. Meanwhile a central cavityj^^thc segmentation cavity, 

 forms, about which as a wall the cells are arranged. 



6. A continuation of the process of cleavage results in the 

 formation of the coarse morula stage, and ultimately there is 

 reached the fine morula or hlastula stage in which the single 

 layer of cells surrounding the segmentation cavity persists. 

 These stages are in the form of a hollow sphere but may be deceiv- 

 ing in some preparations, as the walls often collapse. Carefully 

 focus on one of these balls until the appearance is the same as 

 if a section had actually been made through the center of the 

 blastula. The blastula then appears as a hollow hemisphere of 

 cells, the layer being one cell thick, surrounding the large cavity. 

 Such a view is termed an optical section. 



7. Cell division continues but the cells begin to show their 

 differentiation and shift in position. The cells in one half of 

 the hollow sphere come by invagination to lie within those of 

 the other half. It is analagous to pushing in one side of a hollow 

 rubber ball. Thus an embryo is formed with a body wall of 

 two layers of cells. Study as in optical section. The outer 

 layer of cells is called the ectoderm; the inner layer, which has 

 invaginated, the entoderm; the space resulting from invagination 

 is the archenteron or primitive intestine; and the opening of the 

 archenteron to the outside is the blastopore. This stage is called 

 the gastrula since the alimentary canal is here first formed. The 

 next stage in development is the formation of a third layer of 

 cells, the mesoderm, between the other two. These three layers 

 are called germ layers and from them all of the tissues and organs 

 of the body are derived. 



On the plate showing some of the early stages in the develop- 

 ment of the starfish, first give a proper heading to the plate, 

 then supply the appropriate title for each figure and label all 

 structures discussed in the outline. 



Development of the Frog. — The common leopard frog lays its 

 eggs during April and May in the shallow water of ponds 

 and the margins of lakes. All the eggs are laid at one time 

 in a single complement, each egg being surrounded by a 

 gelatinous envelope. 



After fertihzation by the spermatozoa of the male, which are 

 cast over the eggs as they are extruded by the female, cleavage 



