A DESCRIPTIVE SKETCH 33 



growth, morphological difTerentiation runs its course, and results 

 in the placing of material in particular geometrical relations, roughly 

 in the form and position of the various organs which are to arise. 

 These simple rudiments then undergo growth at particular rates, 

 which rates may be proportional to that of the whole embryo, or 

 faster, or slower. It is obvious that the rate of growth of any 

 particular rudiment relative to that of its neighbour, and any differ- 

 ence in the rates of growth of any one rudiment in the three dimen- 

 sions of space, contribute essential factors in determining the final 

 form of the organ and of the embryo as a whole (see pp. 225, 366). 



§7 



After the position and form of an organ has been roughly blocked 

 out, there follows the process of elaboration of the cells of the organ 

 for the function which they are to undertake in the organism. This 

 is the process of histological differentiation, or histo-differentiation^ 

 as it may be more briefly styled. As a result of this process the cells 

 of the neural tube, for instance, become diversified into supporting 

 or ependyma cells and into neuroblasts, which latter produce axon- 

 fibres and give rise to the tracts of the central nervous system and 

 to the ventral nerve-roots. The dorsal nerve-roots are formed as 

 a result of the production of fibres by the cells of the neural crests. 

 In the eye, the various layers of the retina are very early differenti- 

 ated from one another. Similarly, the cells of the myotomes become 

 differentiated into fibres of striated muscle ; mesenchyme cells in 

 particular regions produce cartilage; others elsewhere produce 

 connective tissue, and others again eventually give rise to bone. The 

 cells of the hypophysis, which comes into relation with the floor of 

 the fore-brain or infundibulum to form the pituitary body, become 

 differentiated into the glandular elements characteristic of that body. 

 Thus, in every rudiment, the cells undergo specialisation to form 

 characteristic tissues, differing from one another and from the 

 simple undifferentiated blastomeres from which all the cells of the 

 embryo arose. When histo-differentiation of an organ has ap- 

 proached completion, the organ is able to enter on a new phase of 

 its development, viz. that of functional activity. Up to this point 

 development has proceeded without function of the organs : indeed, 

 they did not exist at the start and have had to be made. After this 

 HEE 3 



