WEYSSE. — BLASTODERMIC VESICLE OP SUS SCROFA. 315 



having so marked a resemblance to the overgrowth iu Amphioxus 

 has been recorded in any intermediate group. I am tempted, how- 

 ever, to call attention here to a parallel case. The early cleavage in 

 mammals, as described especially by Tafani ('89), bears a striking 

 resemblance to the early cleavage of Amphioxus. Why, then, do we 

 not find the same type of cleavage occurring in intermediate groups 

 of vertebrates? Here asfain it must be admitted that we do not 

 find it, but iu this case the reason is plain ; it is because the whole 

 structure of the ovum is so changed by the accumulation in it of 

 nutrient material, that cleavage on the type of Amphioxus is impos- 

 sible, and it is not until we reach the Mammalia that the conditions 

 are such as to admit of total equal cleavage. But is it not for the 

 same reason that the steps which lead up to the formation of the neu- 

 ral tube have been modified, and that the overgrowth in Amphioxus 

 finds its first striking recurrence in the mammalian embryo ? The 

 lumen of the canal is often extremely small in other mammals, and 

 has in some cases been represented by a single line, e. g. Robinson 

 ('92, Plate XXIII. Fig. 13 A, Plate XXIV. Fig. 15 D, and Plate 

 XXVI. Fig. 17). 



A more serious objection exists in the fact that the cavity beneath 

 the overgrowth in Amphioxus becomes the neural canal, whereas, as 

 I have traced it in the pig, it becomes obliterated by a fusion of the 

 bridge with the ectoderm of the disk. I would not be understood as 

 considering the canal which I have compared to the neurenteric 

 canal decisive evidence in favor of this theory ; it is, however, a sig- 

 nificant phenomenon occurring in a significant position. To my mind 

 it occupies the position of the neurenteric canal in Amphioxus ; that 

 it is certainly the neurenteric canal of the pig, I would not presume 

 to say ; the descriptions of this canal in mammals are varied ; the 

 bridge and canal which I have described have never before been 

 recorded in the Mammalia, so far as I am aware. 



3. Summary. 



My conclusions as to this bridge may be briefly summarized as 

 follows. Two interpretations of the structure have presented them- 

 selves as in some measure probable. The first homologizes it with 

 the thickening of the " Deckschicht " or Rauber's layer, or, as 

 Heape ('83) calls it in the mole, the outer layer, which through the 

 development of a secondary cavity becomes separated from the true 

 ectoderm of the germinal disk and forms a sort of roof over it. 

 That this structure is not homologous with the bridge seems to me 



