588 WILSON. [Vol. VIII. 



larva exactly agreeing, except in size, with the normal larva up 

 to the period when the first gill-slit is formed (Plate XXXVIII, 

 Figs. 136, 138). In many cases, however, the tail is abnor- 

 mally modified, posterior to the anus. 



2. Displacement, without actual separation of the blasto- 

 meres of the 2-celled stage, may give rise to double embryos 

 of many varieties, which may live up to the period of the first 

 gill-slit. In the gastrula stage almost every possible transi- 

 tion occurs between forms slightly expanded laterally (Plate 

 XXXIV, Fig. 64) and those in which two bodies are joined 

 by only a narrow bridge of tissue (Fig. 6"^). The axes of the 

 two bodies may form any angle. The blastopores, if separate, 

 may be turned in any direction, and a great variety exists 

 in the relation of the germ-layers of the two bodies (Plate 

 XXXIV). 



3. An isolated ^ blastomere may undergo a cleavage nearly 

 or quite identical with that of a normal ovum, but often varies 

 more or less widely from it. It may give rise to a ^ blastula 

 and 3^ gastrula, differing from the normal only in size. The 

 segmented stage, with a notochord, is rarely attained and no 

 normally constituted ones were observed. In a single, isolated 

 instance a ]{ larva was obtained (Fig. 139), at a stage nearly 

 corresponding with that of the first gill-slit. This larva pos- 

 sessed a nearly normal notochord, neural tube and neuropore, 

 mesoblastic somites and prae-oral pit, but had no mouth, no 

 gill-slit, no anus, and the posterior region of the intestine was 

 aborted. 



4. A 4-celled stage, if separated into two pairs of cells, as 

 often happens, may give rise to two perfect, half-sized dwarfs 

 (2/4 embryos), 



5. Incomplete separation of the blastomeres of a 4-celled 

 stage gives rise to (a) double embryos, (b) triple-embryos, one 

 body being twice the size of the other two (Figs. 61, 62), and 

 (c) very ^.arely to quadruple embryos. The double and triple 

 embryos may attain the gastrula stage, but I have never 

 brought the quadruple forms beyond the blastula. In the case 

 of triple embryos one body is obviously a 2/4 embryo, produced 

 from one pair of cells, while the others are fourths. 



