214 Prof. E. Warren. On the Development of the 



which will form muscle, are considerably larger than the rest of the 

 mesoderm cells and stain more deeply ; they become arranged in 

 parallel cords. By the arrangement of the primitive muscle, the 

 segmentation of the abdomen is marked out quite early in the life of 

 the nauplius. 



The cells which will form the heart, can be distinguished at an 

 early period. In the thoracic region, the dorsal portion of the mesoderm 

 bands consists of two closely applied layers of flattened cells (fig. 3). 

 These layers gradually grow up over the yolk-sac, and those of one side 

 meet their fellows of the other side in the mid-dorsal line. Separation 

 of the two layers now occurs, and the sac thus formed is the heart 

 (figs. 4 and 5). The pericardial space originates by two processes — 



Fig. 4. — A. Longitudinal vertical section through the dorso-lateral swelling ; it is- 

 taken at some distance from the mid-dorsal line (see fig. 2). 

 B. Similar section taken close to the mid-dorsal line. 



1) the gradual separation of the ectoderm from the heart-sac, and (2) the 

 disintegration of the deeper layers of this thick ectoderm (figs. 2, 4, 5, *). 

 There appears to be a definite floor to the pericardial space, consisting 

 of flattened cells continuous with those of the heart (fig. 5, B), but 

 the roof would seem to be simply the general dorsal ectoderm of the 

 thorax. 



