94 JOHN WAREEN 



■ Jj.. Conclusions 



1. The primary arches can be demonstrated in early human 

 embryos from 10 to 15 mm. in length. 



2. Of the embryos of 15 mm. and over examined in preparing 

 this paper there were about thirty in which the brain was in 

 suitable condition to warrant making observations and in addi- 

 tion to these a number of others were studied but excluded on 

 account of injury or distortion of the forebrain. In the thirty 

 specimens only eight showed any possible signs of a paraphysis 

 and most of these were mostly rudimentary in character. By 

 counting every possible case we get a result of 27 per cent. 

 The fact remains, however, that the structure can be found in 

 human embryos, though in a rudimentary and inconstant 

 condition. 



3. The so-called postvelar tubules or diverticuli can be clearly 

 followed in every degree of complexity in embryos of 19 mm. up 

 to 44 mm. and appear in every specimen studied in those stages. 

 They begin at the diencephalic lip of the velum, have definite 

 limits and involve a relatively short extent of the oral end of 

 the diencephalic roof plate. They always appear as outgrowths 

 from the brain roof and are to be distinguished from ingrowths 

 due to plexus formation. 



PART III. MAMMALIAN EMBRYOS IN GENERAL 



As was mentioned in the introduction, it is intended to review 

 generally the main features of the development of the pineal 

 region in the other mammalian embryos which could be exam- 

 ined in the Harvard Collection. Of these only a few specimens 

 of opossum, cow, deer, and dog embryos were available and the 

 statements on them must necessarily be incomplete. 



1. Marsupials 



Opossum. In an opossum embryo of 11 mm. (fig. 23) the 

 primary arches are all recognizable. The paraphysal arch is 

 short and is succeeded by a very low velum. The postvelar 



