(56 ) 



these nuclei made the impression of a solid indentation. But upon 

 closer examination I always found that this was only an apparent 

 and no real indentation (infundibulum). Here one must be very careful 

 not to draw any conclusions from a few series of sections. In a 

 median section through the infundibular organ from one of my 

 longitudinal series of a 47 mm. long Branchiostoma one would be 

 inclined to draw the conclusion that there exists a groove-shaped 

 indentation of the brainwall behind the organ, no trace of any 

 indentation being found in front of it. So I think it dangerous to 

 found a homology on this indentation, as Kupffer did, and I adhere 

 to the denomination "infundibular organ", as its structure and develop- 

 ment have more resemblance with the epithelium in the saccus 

 vasculosus of the ichthyopsidae, to which I gave the same name, 

 than with the tuberculum posterius, which is still somewhat pro- 

 blematical. 



b. Shape and development of the brain-ventricle. 



I will here only mention those facts that are important for the 

 comparison between the Branchiostoma cerebrum and that of the 

 craniotes, and for the question whether the differentiated epithelium 

 mentioned above may be homologised with the infundibular epithe- 

 lium, or with the tuberculum posterius. 



The second homology might be concluded from the drawing pu- 

 blished by Kupffer in 1894 and 1903, representing a median section 

 through the cerebrum of a 2 cm. long amphioxus. But this drawing 

 seems to me not to represent the real state of things. Neither exactly 

 orientated median sections (fig. 8) nor the median sections recon- 

 structed from series of cross-sections (fig. 7) ever gave me anything 

 like this drawing. 



And yet it is in this case that the reconstruction-method must 

 give an absolutely certain result. By this method we are able to 

 correct entirely the deviations of the cerebral axis from the longi- 

 tudinal axis of the body as they are found in almost every specimen. 

 And as the cerebral vesicle has such a simple uncomplicated form 

 this method gives us in every case an exact reproduction of the 

 median section (which is certainly of the high value for the com- 

 parison of the brain Kupffer ascribes to it), and at the same time 

 allows us to get a sure knowledge of the width of the cerebral 

 cavity. I give here three drawings of the median sections recon- 

 structed from the cross-sections, one of a very young larva of 

 3,4 mm. (fig. 5), one of a young amphioxus of 10 m.in. (fig. 6) 

 and another of a specimen 21 m.m. long (fig. 7). All these are 



