FOEMATION AND FATE OF THE PEIMITIVE STREAK. 459 



description 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 primi- 

 tive streak, but she does not say whether the fusion is primary 

 or secondary. There is, however, no doubt about tbe formation 

 of the area called primitive streak by Schultze in his de- 

 scription of Ran a fuse a, for he describes the process in the 

 following words: — '^ Wahrend auf den vorhergehender Ent- 

 wicklungsstadien dorsal und median das aussere Keimblatt 

 durch einen Spaltraura von dem mittleren Blatt getrennt war, 

 und beide Blatter nur an der ringformigen Zone der Blasto- 

 poruslippe in einander ubergingen, bildet sich gegen Ende der 

 Gastrulation, von der Mitte der dorsalen Urmundlippe aus, cine 

 nach dem Kopfe hin vorwarts schreitendelineare Verwachsung, 

 des Ausseren und mittleren Blattes aus ^' . . . . " diese 

 lineare Verwachsung, in welcher wie in dem Primitivstreifen 

 des Hoheren Wirbelthiere Ektoblast und Mesoblast zusammen- 

 hangen^ 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 statement incompatible with 

 the concrescence theory, which necessitates that "the ento- 

 dermal 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 over- 

 spreads 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 ex- 

 ceeds in length the diameter of the primitive blastopore. He 

 figures no sections to show the structure of the streak, and we 



