44 H, V. NEAL 



with coagulation threads, which are everywhere present," but, 

 unHke Held, finds no genetic relations between such coagulation 

 threads and the elongating neuraxon process. Later ('07) Harri- 

 son writes that "it is by no means certain that the plasmodesms 

 are not artifacts — products of coagulation," and von Lenhossek 

 ('06) offers the same criticism. 



Such criticisms suggest to Held ('09) that Cajal and Harrison 

 have not seen the true plasmodesms, which are not only ver}' 

 easily ruptured but appear everywhere different from coagula- 

 tion threads. They may be covered with coagulation bodies, 

 or not; that varies with individual preparations. If, however, 

 one has ever seen these fine plasmodesmata branch out in char- 

 acteristic fashion from subdividing and branching cell processes, 

 it is impossible longer to hold the view that these fine and defi- 

 nitely branching threads are coagulation products or artifacts. 



Held emphasizes the delicacy of the plasmodesmata and the 

 ease with which in fixation or imbedding and sectioning they 

 may be ruptured and lost. While this is unquestionably true, 

 it is also a fact that the plasmodesms become more conspicuous 

 in preparations in which there has been excessive plasmolysis 

 and vacuolation, as in many preparations made for special neu- 

 rological purposes. On the other hand, the more faithful the 

 cytological fixation, the less conspicuous becomes the coagulable 

 su?jstance found in the intercellular spaces where the neuraxon 

 processes of medullary neuroblasts and those of Rohon-Beard 

 cells grow towards their end-organs. 



While Held (p. 297) admits that Harrison's experiments prove 

 that processes may grow out from neuroblasts in suitable fluids, 

 that nerve elongation may occur exclusively by protoplasmic 

 movement and therefore does not necessarily involve the par- 

 ticipation and differentiation of plasmodesms, nevertheless he 

 maintains that a normal nervous system oriented to the ter- 

 minal-organ is formed only with the participation of plasma 

 connections already present. If, however, it may be assumed 

 that the experiments of Harrison ('04-'ll), Lewis ('06, '07), and 

 Burrows ('11) throw any light upon normal processes of histo- 

 genesis — a conclusion which Held does not seem willing to admit 



