The Microscope. 29 



eggs with a more or less regular segmentation the result of the 

 division is the formation of a mulberry-like mass, a spherical 

 mass of little spheres variously flattened by mutual pressure, 

 the morula. At first the morula is solid, but soon it grows 

 larger, and a cavity is formed in the interior. This is the seg- 

 mentation cavity, and by its appearance the morula is trans- 

 formed into a blastula or blastosphere. 



In the morula condition the egg is in a stage about on a 

 level with some of those not yet thoroughly understood proto- 

 zoans, like Actinophrys, but after we leave the Protozoa there 

 is nothing like it to be found in the whole animal kingdom. 

 All other animals are formed of various layers and tissues, but 

 the morula or blastula is composed of but a single layer of cells. 

 The next series of changes will now be described. 



d. Invagination, and the Formation of the Germinal Layers. 



At the time of the formation of the morula and blastula 

 some embryos (as those of some sponges) leave the egg mem- 

 brane and float freely in the water, swimming about by means 

 of slender vibratile hairs (cilia they are called), borne on the 

 cells. In the great majority, however, the embryo does not 

 escape from the egg until a much later date in development. 



As mentioned above, in eggs with an unequal segmentation 

 we have from almost the first a differentiation of the cells into 

 two groups, one smaller and containing but a comparatively 

 slight amount of deutoplasm, those of the other group being 

 larger and composed largely of food yolk. Theoretically this 

 differentiation should not take place until a later date, and 

 then should be brought about in the following manner : 



To quote an oft-used illustration, the blastula resembles a 

 hollow rubber ball, the cells of which it is composed corre- 

 sponding to the rubber shell, and the segmentation cavity to 

 the hollow of the ball. After the blastula stage is reached, the 

 next in sequence is the formation of the gastrula. Taking our 

 rubber ball, we can force in one side so that the result is a shal- 

 low cup with double walls. Just so in the egg, one side is 

 pushed in, encroaching on the segmentation cavity, and the 

 resulting cup is composed of two layers of cells, and we have 

 the first differentiation of tissues. Each of these layers has its 



