Manchester Meinohs, Vol. l. (1906), No. 3. 23 



With the overgrowth at the Hps of the blastopore the 

 material for the germ-layers is laid down. The super- 

 ficial layer is now the ectoderm. The mesoderm consists 



Fin. 10. 



A series of transverse sections through the blastoderm of ScyUitim in 

 the stage represented in Fig. 7 C to show the formation of notochord and 

 mesoderm. (Original.) 



A passes through the caudal swellings : the mesoderm is seen arising 

 from the roof of the archenteron. 



B. Just in front of the dorsal lip. Two mesoderm plates are now 

 separated from the roof of the archenteron except at the lateral lips (/./.). 

 Medially the roof of the archenteron is thickened for the notochord. 



C. Further in front. The archenteron is divided into a median 

 embryonic portion and two lateral portions beneath the lips of the blastopore. 



D. E. The notochord is cut out of the roof of the archenteron ; the 

 mesoderm is separated into two median embryonic {e.m.) and two lateral 

 extra-embryonic {ex.m.) sheets. 



of two parts (Fzo^. 10), (i) two sheets of cells lying one on 

 each side of the median line over the embryonic portion 



