104 NO FIBROUS CONNECTIVE IN INSECTS. 



nerves and blood-vessels. These were very close to 

 the cells at the earliest periods of development, and a 

 verj close relationship between them must be main- 

 tained throua'hout life, or the free action of the ffland 

 would be impaired. Moreover, as the gland which 

 already actively performs its functions grows, new 

 nerve fibres and new capillaries must be developed 

 around the tubes. The position which a capillary or 

 or an ultimate nerve fibre occupies at an early period 

 will at a later time be the situation where a bundle of 

 nerve fibres, or small arteries and veins must be placed. 

 The structural changes involved in all these altera- 

 tions are considerable. Old capillaries and nerve 

 fibres must be removed as new ones are developed to 

 take their place, and all the original gland cells will 

 have disappeared probably long before the uriniferous 

 tubes have acquired their fully formed characters. 

 But these structural elements are not completely re- 

 moved. There remains a small quantity of matter 

 which cannot be taken up by the ordinary processes 

 at work. This is no doubt capable of being removed 

 like every texture in the body, but its complete re- 

 moval would probably involve the destruction of the 

 gland, while its almost complete removal permits of 

 the continuous development of the latter, and does 

 not interfere with its continuous action. The con- 

 ditions of existence in the case of man and the 

 higher vertebrata, with a few unimportant excep- 

 tions only, permit the very gradual but not absolutely 

 complete removal and renovation of the tissues first 

 foi-med. Hence, as we grow older the greater is 

 the amount of connective tissue that accumulates. 



153. No fibrous connective in insects. — In insects, 

 the state of things is very different, and in their 

 textures there is an almost complete absence of con- 

 nective tissue. The organs and tissues of the larva 

 are entirely removed, while new organs and textures 

 of the imago, or perfect insect, are laid down afresh 



