Geo. S. Huntington 165 
of the fixed sternal line of origin in the differentiation of the muscles. 
This process, in passing from the stage illustrated by Nycticebus to that 
obtained in Hapale, may be regarded as presenting the following steps: 
1. The advantage of the fixed sternal origin leads to the complete 
differentiation of the ectopectoral from the entopectoral as seen in 
Hapale—compared with the incomplete segmentation observed in Nyc- 
ticebus in the caudal common portion of both planes. With this com- 
plete differentiation the volume of the ectopectoral increases and the 
muscle obtains a distinct caudal limit. 
2. The entopectoral likewise begins to differentiate as a stronger 
cephalic portion with sternal origin, leaving the interval seen in Hapale 
between its own caudal margin and the slip arising by an aponeurotic 
lamella over the costal cartilages of the lower true ribs. 
3. The abdominal pectoral appears to be a derivative of the pannicular 
muscle. In Nycticebus, where the pannicular plane is well developed 
and independent, the abdominal pectoralis is wanting. On the other 
hand in Hapale, which animal presents a marked reduction of the cuti- 
cular thoraco-humeral muscle, the abdominal pectoral is fully developed. 
Comparison of the two types produces the strong impression that the 
abdominal portion develops in the lead of the axillary arch. As seen in 
Nycticebus this connection of the pannicular muscle with the Pectoral 
system is well developed. Mesal extension of the same would lead to 
the development of the abdominal pectoral as a secondary derivative of 
the pannicular layer. It is quite possible that, when once established, 
this connection with the main pectoral system would continue to favor 
the more independent development of the abdominal pectoral and cause 
its more complete differentiation from the rest of the lateral thoracic 
panniculus, which in the floor of the axillary space metamorphoses into 
fascial layers, while the original point of connection between the cuticular 
system and the Pectoralis remains as the apparently isolated axillary 
arch. In this way the condition encountered in the majority of the lower 
monkeys (Pl. IV and V, Figs. 6-8) would be explained, where, with a 
well developed and distinct abdominal pectoral and axillary arch, the 
intervening fascial floor of the axillary space is devoid of muscular 
fibres. Many of the human muscular variations, as the different types 
of union between a Pectoralis quartus and the axillary arch, and a num- 
ber of axillary supernumerary muscular slips, lead back to this same 
hypothetical primate condition. An example of these variations occur- 
ring in one of the lower Primates is offered by the abnormal arrange- 
ment of the Pectoral muscles in the specimen of Macacus cynomolgus 
shown in Pl. IV, Fig 5 (Columbia University Museum, No. 1132+). 
