Embryonic Development; Differentiation of Tissues - 277 



technically an asexual process, bears a very at or near the equator, and now the embryo 

 close relationship to the sexual modes of consists of eight fairly equal-sized cells — four 



reproduction. 



DEVELOPMENT-THE EMBRYONIC PERIOD 



During development, the fertilized egg un- 

 dergoes many changes. The original cell 

 multiplies, and the many resulting cells ar- 

 range themselves and undergo differentia- 

 tion — forming the tissues and organs of the 

 growing body. Collectively these complex 

 transformations make up development. Until 

 the time of hatching, or birth — that is, until 

 the offspring emerges from the egg or 

 from the uterus — the developing individual 

 is called an embryo. In detail, embryonic 

 development varies widely in different spe- 

 cies; but certain broad fundamental resem- 

 blances can be observed in the early develop- 

 ment of practically all multicellular animals. 



Cleavage— Formation of the Blastula. 

 Soon after fertilization the zygote divides by 

 repeated mitosis, forming a group of smaller 

 cells. These early divisions, which lead to 

 the formation of a multicellular embryo, are 

 referred to collectively as cleavage. 



Homolecithal eggs (Fig. 15-1 A), which dis- 

 play a homogeneous cytoplasm and relatively 

 little yolk, nevertheless possess a distinct 

 polarity. The animal pole is marked by the 

 point where the polar bodies were formed 

 (Fig. 15-2); and the opposite pole is referred 

 to as the vegetal pole. The first and second 

 cleavages pass through the poles, giving rise 

 to a four-celled embryo (Fig. 15-2). The third 

 cleavage passes at right angles to the first two, 



in the animal hemisphere and four in the 

 vegetal hemisphere. The further cleavages 

 are usually synchronous so that the number 

 of cells increases in geometrical progression 

 — 8, 16, 32, 64, etc. When several hundred 

 cells are formed, the embryo typically has the 

 form of a hollow sphere. This is the blastula 

 stage; and the cavity within the blastula is 

 called the blastocoel (Fig. 15-7). 



Telolecithal eggs (Fig. 15-1B) have an 

 abundance of yolk, especially in the vegetal 

 hemisphere, and the cleavage divisions of 

 the telolecithal egg are not equal. The first 

 horizontal cleavage passes above the equator; 

 consequently, the cells in the animal hemis- 

 phere are distinctly smaller than those in the 

 vegetal region. The lower cells contain a 

 larger amount of yolk and divide less rapidly 

 than the upper cells. Accordingly, when the 

 blastula is formed, the cells in the vegetal 

 hemisphere are larger and less numerous 

 (Fig. 15-7). Also the blastocoel is small and 

 lies eccentrically, nearer the animal pole 

 (Fig. 15-7). In very heavily yolked eggs, such 

 as the eggs of birds and reptiles, the vegetal 

 cytoplasm may not cleave at all; and in such 

 cases the blastula consists of a small disc of 

 cells, roofing over a narrow blastocoel, in the 

 region of the animal pole (Fig. 15-7). 



Gastrulation— Formation of Endoderm. A 

 few simple colonial organisms do not pro- 

 gress beyond the blastula stage. In Volvox, 

 for example, the adult colony consists merely 

 of a hollow sphere of cells, among which 

 very little differentiation has occurred. 



HOMOLECITHAL 

 TYPE 



MODERATELY 

 TELOLECITHAL 



EXTREMELY 

 TELOLECITHAL 



CENTROLECITHAL 



Fig. 15-7. Blastulas derived from different types of egg. Compare with Figure 

 15-1. 



