242 LECTUEES ON BIOLOGY 



Let us imagine the earliest developmental stage of an echino- 

 derm : for instance, the Sea-urchin. The egg-cell, which has 

 been liberated into the sea-water and there been impregnated, 

 divides at first into two cells, and these again into four, eight, 

 sixteen cells, and so forth. In this -manner a large " cell- 

 heap " is produced, the morula. The individual citizens of 

 this primitive cell-state are as yet equally well constructed, 

 no one cell being favoured to the disadvantage of another cell, 

 but this state does not remain long. As development progresses 

 the embryonic cells multiply at an increasing ratio, the individual 

 parts of the cell-heap become more closely united, and from the 

 morula originates the germinal vesicle, the blastula, a hollow 

 sphere the wall of which consists of a single layer of firmly 

 compressed cells. 



That the single parts of the cells are developed differently is 

 clear from the fact that the outer side of the cells, coming into 

 immediate contact with the surrounding sea- water, is exposed to 

 influences very different from those which affect the inner side, 

 which is opposite the ' groove-cavity.' So far the conditions were 

 the same for all parts of the blastula, but now a change takes 

 place. Some cells separate from the rest and wander into the 

 groove-cavity. Then the little hollow sphere invaginates in 

 the manner of an india-rubber ball the wall of which is being 

 pressed at one point towards the other side, and the one-layer 

 germ, the blastula, has become a two-layer germ, the gastrula. 



The necessary consequence of this change is a different 

 development of the cells, corresponding to the influences that 

 act upon them, and to the differences of their functions. But 

 I can only deal with it here in large outlines. The external 

 cells undertake the task of locomotion and form cilia ; the inner 

 cells opposite the cavity, the primary archenteron, undertake 

 the work of nutrition and are suitably transformed ; finally those 

 cells which we saw enter the groove or segmentation cavity 

 produce by continued division and differentiation the connective 

 tissue, the supporting matter, and muscles. In the growing 

 organism division of labour becomes more and more perfect. 

 The individual citizen of the cell-state does not, like the 

 protozoon, care only for his own well-being, but places a part 

 of his strength and work at the disposal of his comrades, the 

 community, which in their turn undertake many of his cares. 



