112 The Philippine Journal of Science i9i6 



pletely separated from each other. In the other three sets the 

 embryos are joined along their lateral sides and would in all 

 probabilities have developed into joined twins (Plate I, figs. 1 

 and 2, Plate II, and Plate III, fig. 1). In the remaining set the 

 embryos approximate each other at the cephalic end (Plate III, 

 fig, 2), but are separated from each other by a deep depression 

 or groove. In the descriptions of these twins "median and 

 lateral" will be used as if we were dealing with only one 

 organism. "Lateral" will refer to the left side of the left com- 

 ponent and the right side of the right component. "Median" 

 will refer to the left side of the right embryo and the right side 

 of the left embryo. 



SET I 



These two embryos are identical in their development. The 

 somites are rather indistinctly marked, but three or four somites 

 can be made out. The heads of the embryos approximate each 

 other along the lateral side, while the caudal ends are more 

 separated, the two embryos forming an angle of about 32 de- 

 grees. Plate III, fig. 1, represents a dorsal view of the embryos. 

 During the fixation the caudal portion of the embryos separated 

 along the medullary grooves, forming a large, artificial gap in 

 each embryo. The head processes are well developed and ex- 

 tend for a short distance over the proamnion. They are well 

 separated from each other by a deep groove. Immediately pos- 

 terior to the second dilatation of the medullary grooves the 

 ectoderm is continuous from one embryo to the other. The 

 medullary grooves are open in their entire extent. In the cepha- 

 lic regions they have undergone differentiation and the first and 

 second primary brain vesicles are formed. Between the two 

 embryos there is an opacity due to the clumping of cells where 

 the germinal layers from each embryo have met. On the ventral 

 side of each embryo the archenteron is prolonged into the head 

 process to form the fore-gut. The area pellucida is well defined 

 ■ and surrounds the embryos uniformly. Some blood-islands have 

 developed in the area opaca. 



SET II 



In set II the components are joined along the thoracic and 

 caudal regions ; this type is known as thoracopagus. The heads 

 are separated from each other, forming an acute angle. The 

 head processes are poorly developed, and the medullary folds are 

 very large and have not fused at any point. Each embryo has 

 an individual medullary groove in the region of the head ; poste- 

 riorly, however, they meet to form one large, wide groove. 



