182 CHARLES R. STOCKARD 



not only involving the anterior half, but extending into the pos- 

 terior part of the body. Finally, at 180° apart, the two embry- 

 onic shields give rise to two completely separate twin individuals. 

 The accompanying diagram may serve to illustrate the man- 

 ner in which such processes operate. In figure 11 the diagram 

 on the left shows two early embryonic shields arising about 20° 

 apart. When the germ-ring has descended further over the 

 yolk-sphere, the dotted line indicates 'how the two embryonic 



11 



Fig. 11 A series of diagrams illustrating the manner in which the degree of 

 duplicity in embryos is determined by the original distance apart of the two 

 embryonic shields on the single germ-ring. The solid lines indicate the eai'ly 

 germ-rings with the two embryonic shields, and the broken lines show the re- 

 sulting body outlines of the former embryos. The figure on the left has the 

 embryonic shields less than 90° apart on the germ-ring and the dotted outline 

 of the resulting embryo indicates it to be a double-headed specimen. In the 

 central figure the embryonic shields are a little more than 90° apart, and the 

 resulting duplicity extends throughout the upper half of the body. In the 

 figure on the right the embryonic shields are 180° apart, or opposite one another, 

 and two complete twin individuals result, as the dotted lines indicate. 



axes become united or common in the body region, and such a 

 condition would finally give rise to a single-bodied fish with two 

 heads. The middle diagram in figure 11 illustrates similar steps 

 in the history of a 'Y monster,' or individual with two heads and 

 bodies and a single tail. The right diagram of figure 11 shows 

 two embryonic shields arising 180° apart, or opposite one another 

 on the yolk-sphere, each of these has an entire half of the germ- 

 ring to develop from, and complete twins are produced. 



