524 



THE DEVELOPMENT OF THE INDIVIDUAL 



grouped together into a mass which somewhat resembles a mulberry and 

 is called the morula, which is the Latin name for mulberry. 



The Formation of Blastula and Gastrula 



As divisions continue, the cells arrange themselves in the shape of 

 a hollow ball which is known as the blastula. The cavity within the 

 ball is the blastocoel. This ball is usually formed at about the 64-cell 

 stage. During all of these cell divisions there has been practically 

 no growth, and the total mass of cells in the blastula has a volume no 



Zygote 



2-cell stage 



8- eel I stage 



Animal pole 

 Blastocoel 



'Veaetal pole 



Morula 



Blastula 



Ectoderm 



Endoderm 



Archenteron 

 Blastopore " 



Gastrula 



Fig. 33.1. Early embryogenesis of a simple chordate (Amphioxus). The zygote 

 divides and forms 2 cells which adhere. After two more divisions the 8-cell stage re- 

 sults. The next drawing shows the morula at the 32-cell stage. The cells at the 

 animal pole are somewhat smaller than the cells at the vegetal pole. The blastula is 

 shown next with the ball of cells cut open to show its hollow interior. The gastrula 

 is shown in the final drawing ; there has been an invagination at the vegetal pole with 



the formation of the archenteron. 



greater than the one cell of the zygote. If a section is cut through 

 the blastula, it can be seen that the cells at one region of the ball are 

 slightly larger than the cells opposite them. The larger cells represent 

 the vegetal pole; and the smaller, opposite cells represent the animal 

 pole. As division continues, the cells of the vegetal pole begin dividing 

 faster than the other cells and push up inside the ball and form an in- 

 vagination. 



