DAVENPORT: PROCESSES CONCERNED IN ONTOGENY. 181 



"We may recognize, in the second place, certain proc< sses which con- 

 cern the direction which the elongated hody takes and its relation to 

 other bodies of the same or of a different kind. This latter group is 

 the only one which requires further analysis and illustration. Vacuo- 

 lization of cords, such as occurs in blood-vessels, will be considered in 

 Section IV. 



In this section we may consider four processes : (1) tropic processes, 

 including the turning of elongated bodies towards or from any object ; 

 (2) the splitting of such bodies; (3) their anastomosis; and finally 

 (4) their fusion with other organs. 



1. Turning of Elongated Protoplasmic Bodies towards or from an Object. 

 This is a process which has long been recognized by botauists as occur- 

 ring in roots and stems, and as being a response to a stimulus coming 

 from outside the organism. Also among hydroids the position of stolons 

 and hydranths is often clearly determined by external stimuli. Of organs 

 inside of the body, the determination of the direction of growth of nerves 

 has been referred by Herbst to the action of a stimulus supplied by the 

 organ towards which the nerve grows. Other examples of this process 

 are not rare. I will cite a few. Herbst has already referred to the 

 case in Turbellaria where unicellular glands of mesencbymatous origin 

 send out long processes (the necks), which gain their appropriate con- 

 nections with the other organs. So, too, the shell-gland of Crustacea, 

 which arises from an aggregation of mesenchyme (K. & II., p. .*'>77) 

 secondarily sends out a stalk which makes connection at the appropriate 

 place. The principal blood-vessels of Vertebrates arise in the area vascu- 

 losa and subsequently grow into the embryo, following certain prescribed 

 paths (M., pp. 215, 21G). From these vessels others in turn bud out, 

 progressing towards their destined organs. According to Field ('91, 

 Bull. Mus. Comp. Zooh, XXI. 222), the pronephric duct of Amphibia, 

 arising in mesoderm, grows at its posterior end, secondarily fusing with 

 the cloaca. Again, the tubules of the metanephros, according to some 

 authors, bud out of the blind end of the ureter, and in their further 

 growth hit exactly the independently formed Malpighian capsules ly- 

 ing in the mesenchyme. In all these cases we have the elongating 

 body clearly turning towards the object with which it is destined to 

 unite. 



Even under some abnormal conditions we have this process taking 

 place ; for instance, when a parasite lodges in any organ of a Vertebrate, 

 new vessels are formed, which grow out towards the source of irritation. 

 (Cf. Roux, '81, Der Karnpf der Theile, pp. 150, 151). 



