THE DEVELOPMENT OF ORGANISMS 735 



When there is Uttle yolk, as in sea-urchin ova, the segmentation 

 results in a hollow ball of cells, the blastula or blastosphere, the 

 cavity being called the segmentation cavity or blastocoel. This is a 

 common embryonic stage and it should be understood that though 

 it is composed of many cells, it is of the same size as the egg-cell, 

 for the cells have not yet begun to grow. But it is lighter than the 

 egg-cell, having used up part of its reserve material. 



If the segmentation cavity is small (e.g. frog) or absent (e.g. 

 sea- anemones), so that the ball is practically solid, the result of 

 cleavage is called a morula. It presents an appearance like the fruit 

 of bramble or mulberry, the external blastomeres bulging outwards. 

 In many cases about four dozen outer cells are visible. Much must 

 not be made of the difference between morula and blastula, for the 

 solid ball may soon develop fluid in its interior, and a hollow ball 

 may soon fill up its cavity with cells. Both are to be distinguished, 

 however, from a type well illustrated by the egg of the crayfish, 

 and restricted in its occurrence to Arthropods, where the core of 

 the segmented egg is an undivided mass of yolk. It is highly probable 

 that the first multicellular organisms were hollow balls of cells, such 

 as may be seen in Volvox to-day, but it would be straining the 

 recapitulation doctrine to maintain that the blastula of a sea-urchin 

 or a lancelet is a recapitulation of a primitive ancestral Metazoon. 

 After all, there are only a few different ways in which an egg-cell 

 can divide into a coherent integrate, and the blastula illustrates one 

 of these. 



GASTRULA. — In the next chapter the cells continue multiplying, 

 but they also begin to grow, and the result is the establishment of 

 the germinal layers. The simplest form of this is seen in a free- 

 swimming blastula, such as that of sea-urchin or lancelet, where 

 the blastomeres of one hemisphere increase in number more rapidly 

 than those of the other, with the result that the more slowly multi- 

 plying hemisphere is apparent^ dimpled into the other, as we might 

 dimple an indiarubber ball which had a hole in it. Thus out of a 

 hollow ball of cells (the blastula), a two-layered sac is formed- — a 

 gastrula. And when it arises in this way, as in coral, starfish, and 

 lancelet, it is said to be formed by invagination or embole. The 

 mouth of the gastrula is called the blastopore, its cavity the archen- 

 teron. In all cases the archenteron becomes the digestive part of 

 the future food-canal or enteron. 



But when the ball of cells is practically a solid morula, the 

 formation of two germinal layers must be effected in a different 

 way. The smaller, less yolk-laden, more quickly multiplying cells, 

 towards the "animal pole", gradually grow round the larger yolk- 

 containing cells, and a gastrula is formed by overgrowth or epibole. 

 In these or in yet other ways the two germinal layers are estab- 



