XXXIII 



XXXIV 



XXXV 



Ch 



----m 



Xh.. 



----m.. 



XXXVI 



XXXVII 



XXXVIII 



Figs. XXVII XXXVIII. Schematic representations of three stages in the gastrulatiou of an ascidian, of Amphioxits 

 and of an amphibian to show their supposed resemblances in (1) Axial Relations, (2) Closure of Blastopore, (3l Origin of 

 Neural Plate, (4) of Chorda and (5) of Mesoderm. The position assigned to the polar bodies in Amphioxus and the amphib- 

 ian is to a certain extent hypothetical. The head of the arrow marks approximately the anterior limit of the neural 

 plate; the tail of the arrow, the median mesenchyme cells in the posterior (ventral) lip of the blastopore. The meso- 

 dermal cells or areas are shaded by entire or broken lines. 



FlQS, XXVII XXIX. Right halves of bisected gastrulae of Cynthia ; the chorda cells are shaded by coarse stipples, 

 the neural plate cells in. p.) by line stipples. The muscle cells (m.s.) lie lateral to the mesenchyme cells (m'cfa l and by the 

 overgrowth of the anterior (dorsal) lip of the blastopore are separated from the mesenchyme cells anteriorly (XXIX), 



FlGS. XXX XXXII, Right halves of bisected gastrula- of Amphioxus (mainly after Hatschek I. In the two earlier 

 stages the existence and position of the mesoderm is hypothetical, being based upon the conditions found in ascidians. 



Figs, XXX I II -XXX V. Right halves of bisected gastruke of the frog 'mainly after Kopsch I. The areas shaded by 

 stippled lines represent the supposed position of the mesoderm, here covered hv endoderm and yolk. 



Figs. XXXVI XXXVIII. Dorsal views of late gastrnte of ascidian (XXXVI), Amphioxus (XXXVII), and frog 



(XXXVIII), The circles marked 1, 2, 3. 4 indicate successive stages in the closure of the blastopore. The actual position 



of the mesoderm in the ascidian and its supposed position in Amphioxus and the frog is shown by the radiating lines 



around the blastopore. The unshaded area (Ch.) anterior to the blastopore and between the halves of the mesoderm rep- 



nts the plate of chorda cells. 



