90 BULLETIN OF THE ESSEX INSTITUTE. 



crania of some other Ichthyopsida, and an attempt is made 

 to see how far the chondrocranium can be employed as an 

 aid in the classification of these forms. 



The various stages are determined in an arbitrary man- 

 ner by body length, but it is to be borne in mind that there 

 is considerable variation in embryos and larvae of the same 

 size. All stages described have been modelled in wax by 

 Born's method, the models have afterwards been care- 

 fully measured and compared and all exaggerations of 

 proportion due to slight variiitions in the thickness of the 

 wax have been corrected in the drawings. 



Ambltstoma punctata. 



First stage. — Embryo ten mm. long. Mouth on the 

 point of breaking through (Figs. 1 and 2). This stage, 

 which I have taken as the starting point of my investi- 

 gations, corresponds closely with that at which Ph. Stohr 

 begins his classic account of the chondrocrania of Triton 

 and the Axolotl ('79). The chondrocraiiial elements are 

 still in the pro-cartilage condition but they are clearly 

 difierentiated from the surrounding parts by the arrange- 

 ment of their cells. Three pairs of elements are present, 

 namely, the parachordals, trabeculae and the quadrates. 



The parachordals (v) lie as two triangular plates upon 

 each side of the slightly depressed anterior end of the 

 notochord. One side of each triangular plate is directed 

 anteriorly and the antero-exterior angle joins the poste- 

 rior end of the trabecula, Avhile the inner face of each 

 abuts against the side of the notochord (n). The lateral 

 margin of the parachordal runs obliquely outwards and 

 forwards from its posterior angle to the junction with the 

 trabecula. The parachordals arise separately and at this 

 stage touch only at the sides of the notochord, thus leav- 



