Hertwig maintains that each stage in the development is the cause of the next stage, and states that a description of the series of stages through which the embryo passes gives a causal knowledge of the phenomena of development. He claims that beyond this descriptive knowledge we cannot hope to penetrate. Both Roux and Driesch have taken issue with Hertwig, and have pointed out that while each stage in the development contains within itself the causes of the succeeding stage, yet we gain no idea as to these causes from a simple description of two consecutive stages themselves. To state that the fertilized egg is the cause of the cleavage gives us no idea of what sort of a process the cleavage is, or how it arises, or what determines the sequence of the divisions, etc. The blastula, for instance, contains the factors that produce the gastrula; but to state that, in a physical sense, the blastula is the cause of the gastrula is an erroneous interpretation of what is meant by causal knowledge. If Hertwig’s idea were correct, there would be as many causes in each embryo as there are stages in its development, and as many causes in the whole range of embryology as there are forms that develop multiplied by the number of stages in each embryo. What we should seek to discover is the particular cause that brings about each kind of process. If we could discover the cause in one single case, it is highly probable that it would be found to extend to a large number of other cases. Driesch formulated an hypothesis of development in his _Analytische Theorie_, but has modified and changed it in several later contributions. It is difficult to give in a few words the subtile analysis which Driesch has made of the phenomena of development. His analytical theory rests on the dictum that the prospective value of each blastomere is a function of its position in the whole. By “function” is meant “a relation of dependence of a general unknown kind.” This idea is connected with the following one, viz. that any blastomere could be interchanged with any other one without altering the end-result. A few elementary processes are supposed to be “given” in the structure, or in the composition of the egg. Each elementary process is the outcome of a cause, and each elementary process must release the succeeding causes,--_i.e._ if the organization of the phase A is present, one of the causes of the next phase B is also then present. The first elementary process is the cleavage, that is initiated (“ausgelöst”) by the fertilization. After a fixed number of divisions has taken place, the cleavage process comes to an end. It has led to the production of a number of cells having similar nuclei but having a different plasma structure, and the result is the blastula stage. Organs whose formation starts from the blastula stage are called primary organs; the archenteron, the mesenchyme, the ciliated band, and the mouth of the sea-urchin embryo belong to this class.