EXPLANATION OF PLATES IX, X, Xl—coniinued. 



PLATE X. 

 Pig. 15.— Transverse section of the head end of a very young embryo. 

 Fig. 1G«. — The same of an embryo 0*2 mm. in length. 

 Fig. 16^. — Section immediately behind 16*. 



Fig. 17. — Section of the head end of a rather more developed embryo. 

 Fig. 18. — Section of the head end of an embryo 0'23 mm, in length. 

 Figs. 15 to 18. Obj. DD, ocul. 3. 



Fig. 19 a, b, c, d. — Successive sections of the head of an embryo 0'5 mm. 

 in length. Obj. DD, ocul. 1. 



Fig. 20 a, b, c. — Succesive sections of the head of an embryo 0"4 mm. in 

 length. Obj. DD, ocul. 3. 



Fig. 21 a, b, c. — Longitudinal horizontal sections of the head end of an 

 embryo 0"6 mm. in length, going from the ventral to the dorsal face. 

 a is the fifth, b the sixth, e the ninth, and e the tenth of the series. 

 Obj. DD, ocul. 1. 



Fig. 22. — Longitudinal vertical (sagittal) section through the middle of the 

 anterior part of an embryo 0"22 mm. in length. Obj. DD, ocul. 3. 



Fig. 23. — Anterior portion of a longitudinal vertical (sagittal) section of 

 an embryo 0"6 mm. in length. Obj. DD, ocul. 3. 



Fig. 24. — Vertical section in the median line of an embryo more than a 

 railimetre in length. Obj. DD, ocul. 1. 



PLATE XI. 



Fig. 25 a, b, c. — Three successive sections through the posterior part of 

 the _ medullary plate of an embryo 3*0 mm. in length, going from 

 behind forwards. 



Fig. 26 a, b, c. — Successive sections in the same direction, from the an- 

 terior part of the same embryo. 



Fig. 27 a, b, c, d, e. — Successive sections in the same direction, from an 

 embryo of 4-5 mm. in length. All the figures in this plate were drawn 

 with obj. DD, ocul. 3, Zeiss. 



