432 EMBRYOLOGY. 



here also the epithelium of the choroid plexus attaches itself to the 

 surface of the optic thalamus by means of thin bands consisting of 

 medullated nerve-fibres (tseniae thalami optici). 



Finally, out of the hinderinost portion of th< roof of the between- 

 brain vesicle a peculiar organ, the pineal yland (fig. 241 z), takes its 

 origin at a very early period, in Man in the course of the second 

 month. Since in recent years numerous interesting works have 

 appeared concerning it, and since many striking discoveries have 

 been brought to light both in the case of the Selachians and more 

 especially in that of the Reptiles, I will describe it at some- 

 what greater length. 



The Development of the Pineal Gland (Epiphysis cerebri). 



First it is to be mentioned that, with the exception of Amphioxus 

 lanceolatus, the pineal gland (glandula pinealis s. conarium) is not 

 wanting in any Vertebrate. It is in all cases formed in exactly the 

 same way. On the roof of the between-brain, where it is continuous 

 with the roof of the mid-brain or the lamina quad ri gemma, there 

 arises an evagination (figs. 238 and 241 z) which has the shape of the 

 finger of a glove, the processu^ pinealis \epiphysis cerebri], the apex of 

 which is at first directed forward, but subsequently backward. In 

 its further metamorphosis there appear, as far as our knowledge at 

 present extends, differences of considerable importance. 



According to the investigations of EHLERS, the pineal process 

 attains in adult Selachians an unusual length ; its closed end swells 

 into a vesicle, which penetrates the cranial capsule and extends out 

 to the dermal surface. In many Selachians, such as Acanthias and 

 Raja, the vesicular end is enclosed in a canal of the cranial capsule 

 itself ; in others it lies outside between the cranial capsule and the 

 corium. The [proximal] end of the vesicle is united to the between- 

 brain by means of a long slender canal. 



Manifold conditions are met with in Reptiles, as the recent investi- 

 gations of SPENCER have taught. These conditions permit in part a 

 direct comparison with the Selachians, but in part they are widely 

 altered. Here, too, the pineal gland is a structure of considerable 

 length, the peripheral end of which lies far away from the between- 

 brain under the epidermis ; it passes out through an opening in the 

 roof of the skull which is situated in the parietal bone and is known 

 MS thf foramen ixmeMdt. The position of the latter can easily be 

 determined on the head of the living animal, because at this place 



