ox THE PROCESS OF GASTRULATION IX CHELOXIA. 249 



ji deep fiwsiire cutting the yulk-plug into liuJves. I am unable to give 

 any ex[)Janation of thi« groove, whicli is very unusual, this being 

 the sole instance among hundreds of chelonian embryos that have 

 passed through my hands. 1 think it may probably be teratological. 

 Kuptfer (No. G, Taf. IV. Fig. 40 /. & y.) gives two figures of 

 Coluber that are strikingly like this. 



To sum up the facts of Gastriilation as above described : — 



1. When segmentation has gone on for some time, 

 there is established in the blastoderm two layers: (<<) the 

 superficial epiblast composed of columnar cells, and (/)) 

 the lower layer composed of irregular stellate cells and 

 probably not forming a complete sheet. 



2. This separation into two layers takes place in all 

 parts of the blastoderm v\^ith the exception of a small 

 area at the posterior end of the future embryo. Here 

 not only is there no differentiation of layers but a thick 

 knob consisting of a network of cells is produced by the 

 accession of cells from the subjacent bed of yolk. The 

 mass can not be said to belong to either of the two 

 layers above named. This is the Primitive Plate or Prim- 

 itive Knob. 



o. In the middle* of the Primitive Knob, an in- 

 vagination cavity is produced, which at first goes straight 

 downwards but soon takes a forward horizontal course. 

 This is the Jnvagination-Cavity or the Archenteron. its 

 dors;il opening is the Blastopore. The in vagination-ca vity 



* Will (X'u. 21) is uo doubt quite correct in printing out that the invagination -cavity 

 begins much nearer the anterior than the posterior end of the primitive plate. In front of it 

 there is only the future anterior or dorsal lip of the cavity. 



