THE PINEAL SYSTEM OF REPTILES 241 



Development of the Pineal System in Reptiles 



In addition to the pioneer work of Leydig, Strahl and Martin, Beraneck 

 and others, the early stages of development of the pineal outgrowth 

 have been studied by Nowikoff (19 10) in Lacerta muralis and L. vivipara, 

 and by Dendy (1899-1911) in Sphenodon. According to Nowikoff, 

 the first indication of the pineal outgrowth occurs as a thickening of the 

 neural-epithelium forming the roof of the diencephalon. It is situated 

 in the median plane and indicates the junction of the diencephalon with 

 the midbrain. A transverse groove divides it into an anterior segment 

 which will become the parietal eye, and a posterior segment (Fig. 172, A). 

 At the 4-mm. stage (Fig. 172, B) the apex of the evagination is seen to be 

 directed forward, and the groove now forms a constriction which com- 

 pletely surrounds the tubular evagination. In a 6-mm. example of 

 L. muralis (Fig. 172, C) the anterior segment has become detached from 

 the posterior or proximal part, and now forms a closed vesicle lying 

 between the epidermis and the brain. Later in a 9-mm. embryo of L. 

 vivipara (Fig. 172, D) the parietal vesicle is seen to be separated from the 

 roof of the diencephalon by an ingrowth of mesoderm containing blood- 

 vessels. Traversing the mesoderm is a bundle of nerve-fibres which 

 connect the parietal eye with the right habenular ganglion. This lies 

 in front of the posterior segment or pineal sac and it was considered by 

 Graham Kerr (19 19) to have arisen by the development of nerve-fibres 

 in the primary bridge which connected the wall of the vesicle with the 

 roof of the brain, when these were in contact, as shown in Fig. 172, C. 

 The connection of the nerve with the right habenular ganglion through the 

 habenular commissure agrees with the description by Klinckowstroem 

 (1894) of the connection of the " pineal " or parietal nerve in Iguana tuber- 

 culata, but differs from that in Sphenodon which was described by Dendy 

 (1899), in which the connection is with the left habenular ganglion. A 

 thickening of the superficial or distal wall of the parietal organ in Lacerta 

 which was first noticeable in the 6-mm. stage becomes marked off later as 

 the lens (Fig. 172, C), while the remaining portion of the vesicle becomes 

 differentiated into the sensory or receptive part — the retina — the structure 

 of which will be described later (p. 253). At the 25-mm. stage (Fig. 173) 

 the parietal organ becomes further separated from the pineal sac by the 

 interposition of the paraphysis and dorsal sac, around which the greatly 

 elongated pineal nerve courses in its passage from the parietal organ to the 

 habenular commissure. 



Several views have been expressed in explanation of the appearances 

 seen in the early stages of development of the pineal system ; briefly 

 stated, these are : 



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