g CHIM/EROID FISHES AND THEIR DEVELOPMENT. 



We notice, first of all, that the peripheral zone of the blastoderm (between the 

 points a-b) is less complicated than its central portion (between b-c). The periph- 

 eral zone is, however, more highly differentiated than in a similar region in an 

 elasmobranch (cf. Ruckert's memoir in Kupffer's Festschrift, plate vii, fig. 75); 

 witness the definite character of the ectoblast and yolk entoblast, and the gigantic 

 size of many of the mesoblast cells. But it is in the central region of the blasto- 

 derm where the conditions are most extraordinary; we observe, that at many 

 points, 1-5, masses of cells extend downward from the ectoderm, proliferating in 

 ridges, sometimes giving rise to root-like processes. These terminate below either 

 freely, or they may actually fuse with the entoblast; at various points, 6, they he 

 close to the entoblast; at 7 is shown a point where they become continuous with the 

 entoblast (the continuity to be traced in the serial sections). They thus form the 

 spongy meshwork which we have already noted in the surface view of this stage, 

 a condition of complication, which, as far as I am aware, is unknown in the 

 extra-embryonal blastoderm of so early a stage in any other vertebrate. 



We note in connection with the spongy character of the blastoderm the 

 presence of many large cells (unshaded in the figure), some of which, like many m 

 the neighboring spongy trabecular, are undergoing numerous divisions (amitotic) as 

 at 8.* To understand the meaning of this spongy blastoderm one should first 

 consider it in its prospective value. Later specimens show that in this region 

 appear blood-vessels, and in the present early preparation — and even indeed in 

 earlier ones, we are evidently dealing with the beginnings of vascular structures. 

 In fact in the trabecular themselves we find at various points (9) the cells already 

 groui)ed together so as to form cavities, and in the latter large granular cells are 

 undergoing subdivision, in the direction evidently of blood-building. In this character 

 again, it will be remarked there is given an im]jortant instance of the precocious 

 mode of development of Chimsera. In other words, in this form at a period which 

 outwardly suggests stage b of the shark the vascular development in the extra- 

 embryonal blastoderm is (approximately) equivalent to the shark's in stage e. 



*W'e have here again evidence against the commonly accepted view (of Flemming, Ziegler, and von Rath) as to 

 the significance of araitosis. Admitting that these cells come to form blood and blood-vessels, it must also be granted, 

 as the following evidence shows, that the blastoderm becomes part of the young fish, and therefore the behavior of its 

 cellular components is not to be compared with that of the vitellophagous periblast nuclei in the teleost. Of course 

 it will be seen, on the other hand, that the adherent of the Flemmingian view might object that although the blastoderm 

 itself was a permanent structure of the embryo it might none the less contain provisional cellular elements (nutritive). 

 He will admit, however, that this rarified view as to the fate of component elements of the blastoderm receives little 

 support from the examination of related elasmobranchian structures. 



The present evidence, it seems to me, favors the view that amitosis is but a symptom of early and rapid cell- 

 multiplication. Such a need for rapid division often occurs in evanescent structures, and hence it may happen that 

 this type of division has been given less consideration than it is justly entitled to, from the standpoints both of cell 

 physiology and cell philosophy. In this matter I need merely mention, in view of the scope of the present paper, that 

 there is rapidly accumulating a mass of evidence against the decadent character of amitosis. In the nature of such 

 evidence are the observations of Conklin (Am. Nat., Oct., 1903) on the egg follicle cells of Gryllus ; Kellogg's results on 

 similar structures in Hydrophilus (Science, Mar. 4, 1904); also H. L. Osborn's observations on Fasciolaria (Science. 

 Feb. 5, 1904) in which amitosis occurs in stages of gastrulation : Boeke's statements that in teleosts mitotic may 

 arise from amitotic nuclei (Fetrus Camper, vol. 11, Afl. z. pp. i6i, 1902): finally, Child's ".Amitosis in Moniezia" (.\nat. 

 Anz., vol. XXV, 1904). 



