GENERAL ZOOLOGY 



In the amphioxus {Branchiostoma lanceolalum) cleavage is total; that is, the 

 entire zygote is divided into two cells of the same size, and division con- 

 tinues to produce cells that differ but little in size (Fig. 5.145 and C). The 

 cleavage pattern is like that of the frog, in which cleavage is also total but 

 in which inequality in the sizes of cells is soon apparent. In the frog the 

 plane of the first cleavage furrow, as the cytosomal constriction is called, 

 passes from the animal to the vegetal pole. Usually one of the resulting cells 

 is destined to give rise to the right side of the individual, the other to the 

 left side. This fact has been established because of certain changes that occur 

 in the frog's egg after the entrance of the sperm. The sperm enters at some 

 point in the animal hemisphere, and as the second meiotic division occurs, 

 streaming movements in the cytoplasm result in a distribution of material with 

 reference to the plane of entrance of the sperm and the path it follows as it is 

 carried toward the egg nucleus (Fig. 5.15.4 and B). These streaming move- 

 ments of the cytoplasm, in addition to producing invisible localization of 

 cytoplasmic regions, give rise to the gray crescent, an area from which some 

 of the pigment is carried away and which lies approximately opposite the en- 

 trance point of the sperm. With this visibly different region as a landmark, 

 observations can be made concerning the fate of certain regions of the zygote 

 during development. Thus, it has been determined that the future median 

 longitudinal plane of the embryo coincides with a plane passing through the 

 egg axis and bisecting the gray crescent (Fig. 5.15Z?). Since the first cleavage 

 plane usually bisects the gray crescent, it follows that bilateral symmetry 

 becomes apparent at this time. 



The second cleavage furrow likewise appears first at the animal pole and 

 passes to the vegetal pole at right angles to the first, so that a four-cell 

 stage results. These cells are of equal size, but cleavage now becomes 

 unequal in the frog. The third cleavage furrow cuts each of the four 

 cells in a plane parallel to the equator but nearer the animal than the 

 vegetal pole. Of the resulting eight cells, the four in the animal hemisphere 

 are smaller. Two fourth cleavage furrows appear simultaneously in the 

 smaller cells and pass through the larger cells to form a 16-cell stage. After 

 this, two fifth cleavage furrows, one on each side of the third cleavage furrow, 

 produce 32 cells. Indications of an irregularity of division rate can often be 

 seen during the fourth and fifth cleavages, since the furrows pass more rapidly 

 through the cells containing less yolk. 



Formation of the Blastula. As the egg divides during cleavage, the cells 

 tend to become spherical. Since the yolk stored in the female germ cell is 

 being utilized as the source of energy for cleavage, a small cavity appears 

 internally as early as the eight-cell stage in the frog. This cavity is quite 

 conspicuous after the fifth cleavage and is known as the cleavage cavity, 

 blastula cavity, or blastocoel. The developing individual is now called a 

 blastula, and this period in development is referred to frequently as the 

 blastula stage. The blastula arises as a result of cell division only; no cell 

 movements have occurred. If the egg is isolecithal and cleavage is total and 



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