200 Mr. A. Hyatt on the 



primitive type, and tliis is eminently the case with Porifera. 

 We should anticipate the opposite with a hig-her type like 

 the worms or any metameric animal, and this appears to be 

 borne out by what Biitschli brings forward in support of his 

 theory. 



In CuGullanus the earliest stages are rounded, and we can- 

 not agree with Biitschli that the flattened form which follows 

 this is a primitive placula or diploplacula. The primitive 

 placula is a single layer which becomes double or diploplacu- 

 late, and in both stages must precede the morula, and cannot 

 succeed this stage. It will be seen by our remarks above 

 that the esoteric and exoteric differentiations would have 

 occurred normally before the morula stage in the placula of 

 Cucullanus or else in fusion with it, and therefore the double- 

 layered placula of Biitschli would be necessarily a flattened 

 morula in which the two layers had already been formed. 

 The relations of the planula stage in Cucullanus and Lum- 

 hricus to the gastrula also indicate that it is simply a modifi- 

 cation of the morula stage, and not comparable with the 

 earlier premorula stages of the embryo. The formation of 

 the gastrula in Cucullanus is a beautiful example of extra 

 growth of the ectoblast, as has been pointed out by Balfour ; 

 and in this and in Lumbricus a true embolic gastrula is 

 formed by this process, which is not more primitive than that 

 which occurs in the Ctenophoroi or Tuhularice. Tiie gastrula, 

 in other words, is formed according to a highly concentrated 

 secondary mode of development, and not by primitive or 

 simple processes. We should therefore, even while adopting 

 Blltschli's theory, decline to accept his typical examples as 

 true illustrations of the theory, and hold rigidly to the law of 

 succession in the stages of the embryo for justification of this 

 position. 



We cannot give a better illustration of what we mean by a 

 monoplaculate embryo than Hatschek's AtnpMoxus in the 

 four-celled stage *, nor of our diploplacula than the same in 

 the eight-celled stage, when the cells of the esoteric layer are 

 first differentiated, which occurs even before the two poles of 

 the embryo become closed and long previous to the stage when 

 the blastula is formed. 



Immediately after the diploplaculate stage the ovum of 

 Porifera and Amj^hioxus, as well as some other types, presents 

 a stage during which it is a tube open at both ends. The 

 hereditary significance of this stage indicates a tubular ances- 

 tral form, through which water would freely circulate ; and 



* Arbeit, d. Zool. lust. d. Univ. ^Vit■Il, iv. Ileft i. pi. i. 



