FORMATION AND FATE OF THE PRIMITIVE STREAK. 89 



Morgan (39), Alice Johnson (27), and Schultze (45) mention a 

 primitive streak in front of the blastopore. Morgan gives no descrip- 

 tion either of the formation or structure of the streak. Alice Johnson 

 (27) describes a fusion of the germinal layers in the region in front of 

 the blastopore which she calls the primitive streak, but she does not 

 say whether the fusion is primary or secondary. There is, however, 

 no doubt about the formation of the area called primitive streak by 

 Schultze in his description of Rana Fusca, for he describes the process 

 in the following words: — "Wahrend auf den vorhergehender Ent- 

 wicklungsstadien dorsal und median das aussere Keimblatt durch 

 einen Spaltraum von dem mittleren Blatt getrennt war, und beide 

 Blatter nur an der ringformigen Zone der Blastoporuslippe in einander 

 ubergingen, bildet sich gegen Ende der Gastrulation, von der Mitte 

 der dorsalen Urmundlippe aus, eine nach dem Kopfe hin vorwarts 

 schreitende lineare Verwachsung, des ausseren und mittleren Blattes 



aus " " diese lineare Verwachsung, in welcher wie in dem 



Primitivstreifen des hoheren Wirbelthiere Ektoblast und Mesoblast 

 zusammenhangen, von der dorsalen Urmundlippen sich nach dem 

 Kopfe hin allmahlich ausdehnt, und wachst also der Primitivstreifen 

 auch bei den Amphibien von hinten nach vorn " (45, p. 330). 



This statement of Schultze's tends to increase the difficulty of the 

 situation, for it agrees neither with Balfour's nor with the concrescence 

 theory of the primitive streak. In opposition to Balfour's theory 

 Schultze places the primitive streak in front of the neurenteric canal, 

 and he says that the streak is formed from behind forwards, a state- 

 ment incompatible with the concrescence theory, which necessitates 

 that " the entodermal canal and primitive streak begin at the edge of 

 the blastoderm and grow at their posterior end away from the seg- 

 mentation cavity, and at the same rate the blastoderm overspreads 

 the yolk" (36, p. 511). 



Neither do Erlanger's observations (10a) tend to simplify the 

 problem. He associates the primitive streak with the antero-posterior 

 closure of the dorsal portion of the blastopore, but according to his 

 figures the primitive streak ultimately exceeds in length the diameter 

 of the primitive blastopore. He figures no sections to show the 

 structure of the streak, and we are inclined to believe that he has 

 mistaken the neural furrrow for the primitive streak. But if his con- 



