72 K. MITSUKURI; 



lary folds and forms in tlie development of the tn.il its extreme tip, is 

 tenable. It might be said perhaps that the yolk-]jlno' may behave 

 differently in different species. But [ have worked its history out 

 in three species, and it seems incredible to me that in any species it 

 will undergo modifications that seem to liave no genetic relation 

 with tliose I have sketched out above. 



It may not be out of place to summarise here the facts given in 

 the preceding pages, and to give a partial discussion of them. The 

 consideration of their theoretical significance T shall reserve for the 

 next section. 



At the very earliest stage of deveIoj)ment, there is in the posterior 

 part of the embryonic shield an area in which the ]jri]nary layers are 

 not differentiated — -the |)i-imitive plate or knob. When the process 

 of gastrulation begins, it takes ])lace within tliis area. Wlien this 

 has ()C(;urred, all that part of the ])rimitive knob /// front of the 

 blastopore is fnuid to have differentiated itself into the epiljlast and 

 the primary liypoblast — these two layers being reflected into each other 

 at the anterior lip of the blastopore. JU'lu'nd the blastopore, however, 

 there is still an area, in which the lavers are fused and oxer Avhich the 

 epiljlast has not theref)re spread itself. Ijy the gradual change of 

 shape in the blastopore (see Woodcut \'), this posterior part is 

 gradually hemmed in, and becomes surrounded, by the anterior lip 

 of the blastopr)re. Thus, at a certain stage (Figs. 1 it 1((), the blasto- 

 pore has assumed the horse-shoe sha])e and the posterior fused mass, 

 free from the epiblast, is enclosed within the horse-sh(je. This mass 

 stands up, as it has always done, from tlie floor of the invaginated 

 archenteron (or the blastoporic passage, when the archenteron has 

 broken through below; see Contrih. IV). Tliis is the mass wdiich 

 we have called the yolk-plug. Its relations to the germinal la3^ers 



