34 COMPAEATIVE ANATOMY. 



Origin of the Organs. 



§ 28. 



In section 13, the title of organs was given to those parts of 

 the body which were entrusted with a definite function for the 

 purposes of the organism, and which had a form in correspondence 

 with this function. In this general sense every form-element is 

 an organ, just as much as the parts, which are made up of form- 

 elements, and have a definite function, are organs. The conception 

 of an organ is therefore a relative one. We must accordingly 

 separate organs into those of a lower and those of a higher order. 

 The former are represented by the morphological units or form- 

 elements — elementary organs — while the organs of a higher order 

 are those which are made up of a number of elementary organs — 

 cells, and their derivatives (tissues) — and which are set apart for 

 a single function. There are but few of these organs of a higher 

 order in the lowest stages of animal organisation, owing to the 

 simplicity of the organism. But these few organs form the ground- 

 work on which the gradual complication of the organism is raised 

 up by continued differentiation, and in accordance with the principle 

 of the division of labour. We may therefore call those simple 

 organs of a higher order, from which complex organs are developed 

 by differentiation, " primitive organs." 



When we examine these primitive organs more closely, we find 

 it convenient to associate them with the earliest processes of dif- 

 ferentiation which take place in the organism, for they can be 

 derived from them. A collection of smaller cells arises from the 

 division of the egg-cell, and these have not all the same position. 

 Some occupy the inner part of the organism, and others form a 



layer which surrounds it, and at the same time 

 forms the external boundary of the body 

 (Fig. 14). If in this stage of development 

 the taking in of food into the body com- 

 mences, then the inner cell mass becomes con- 

 verted into the limiting layer of the digestive 

 cavity, and forms a primitive gut (enteron). 

 In many observations the process of division 

 into two layers is described as due to the 

 Fig. 14. Separation of invagination of a one-layered vesicle. In 



the mass of cells formed ^ aer cases it is represented as taking place 

 by change of the yolk „ ., , .. r . . ., , °, l 



into a peripheral (c) differently, so that it is impossible to make out 



and a central (d) por- whether there is any phenomenon common 



tl0n - to all cases, and, if so, how far it is common. 



Let us therefore turn to the results of the 



process, without making any generalisation about it. We now 



have an organism made up of two layers of cells. An outer one, 



or ectoderm, which forms the primitive integument, and an inner 



