56 RICHARD EVANt>. 



change, the dift'erentiation that has taken place beyond what 

 was observed in the third embryo being vei-y small, but in 

 spite of this fact, the growth in size of the nervous rudiment 

 is considerable. Most marked of all is the increased thick- 

 ness of the forecast of the brain, which so far shows no sign 

 of demarcation into several ganglia or lobes, and is situated 

 in front of the renal opening of the first somite (PI. 7, figs. 

 12 a, 12 &, 12 c, and 12 d). 



Both the stomod^eal and proctodaeal invaginatious are well- 

 formed structures, and communicate with the irregularly 

 shaped enteron (PI. 7, figs. 12 e and 12 k). 



The mesodermal somites have attained, in a general way, a 

 more advanced stage of development than they had in the 

 previously described embryo, in which they were not divided 

 into appendicular and median portions. In the present 

 embryo the renal portion of the first somite communicates 

 with the exterior, and the same portion of several other 

 somites has reached the ectoderm, though the opening has 

 not been actually formed (PI. 7, figs. 12 d and 12 e, ren. oi^.). 

 " Germinal nuclei " have already appeared in the splanchnic 

 walls of several somites (PL 8, fig. 13). 



In the embryo under consideration the endoderm seems 

 in some respects to be in a less advanced state of develop- 

 ment than in the third one. In connection with its structure 

 there are several points which should be noticed. In the 

 first place, the peripheral endoderm is not so well marked off 

 from the central mass as it was in either the second or the 

 third embryo, and it often contains within its substance a 

 number of yolk-bodies belonging to the type referred to as 

 compound systems, which was not the case in the third 

 embryo (PI. 7, fig. 12/). In the second place, the presence 

 of compound systems marks a decidedly less advanced state 

 of development, unless they are regarded as the products of 

 the metabolic activity of the endodermal elements them- 

 selves, and different from those, occurring in the second 

 embryo, which were directly derived from the yolk-bodies 

 originally found in the egg. It seems that this second alter- 



