6 PRINCIPLES OF EMBRYOLOGY 



supplying these stores of yolk. Finally, and most important of all, it is 

 during this time that the egg-cell acquires its basic structure, which pro- 

 vides the framework for all the elaboration which will occur in later 

 development. This basic structure always involves a polar difference by 

 which one end of the egg becomes different to the opposite end; these are 

 the so-called animal and vegetative poles. There may be also a second 

 difference, distinguishing the dorsal from the ventral side and thus defm- 

 ing a plane of bilateral symmetry; perhaps indeed there is always some 

 trace of such a difference, though it is not always well marked or very 

 stable. Lastly one may mention a difference of another kind, between a 

 cortex which forms the outer surface of the egg and an internal cytoplasm 

 which is usually more fluid. We shall see that all these three elements of 

 structure — the animal-vegetative axis, the dorso-ventral axis and the cor- 

 tex-cytoplasm system — play very important roles in development. 



Stage 2. Fertilisation. This stage involves two important processes; the 

 union of the haploid nucleus carried by the egg with that of the sperm, 

 and the 'activation' of the egg, which causes it to begin dividing and thus 

 to pass into the next stage. These two processes are distinct from one an- 

 other, and we shall see that activation can happen without any union of 

 the nuclei taking place. 



Stagey- Cleavage. The egg-cell becomes divided into smaller and smaller 

 parts by a process of cell division. There are many different patterns in 

 which such cleavage can occur, and it is greatly influenced by the presence 

 of large quantities of yolk. 



Stage 4. The hlastula. Cell division continues throughout the greater part 

 of the embryonic period, but the stage of cleavage is said to come to an 

 end when the next important developmental event occurs. This event is 

 gastrulation, and the embryo which is just ready to start gastrulating is 

 spoken of as a blastula. In its most typical form the blastula consists simply 

 of a hollow mass of smallish cells; these have been produced from the 

 egg by cell division, and the hollow space in the middle of the mass is 

 formed by the secretion of some fluid material into the centre of the group. 

 When there is a considerable quantity of yolk, the blastula becomes 

 asymmetrical, the cells which contain a high concentration of yolk being 

 larger than the others. In the extreme case, such as in the eggs of birds, the 

 yolky end (the vegetative pole) does not cleave at all, and the blastula 

 becomes reduced to a small flat plate floating on the upper pole of the 

 yolk; this is known as a blastoderm. 



