68 LEONARD W. WILLIAMS 



row anteriorly but is broad and shallow posteriorly. The muscle 

 plate underlies the medial fourth or fifth of the upper wall of the 

 somite and its nuclei are gradually becoming elongated in the 

 longitudinal plane, instead of as before in the transverse plane. 

 The boundaries of the cells of the muscle plate are indistinct but 

 are probably present. 



The further description of this somite requires a study of the 

 adjacent blood-vessels which have been best described by Evans. 

 His figure ( '09, 2, fig. 3b) of the blood-vessels of the head of an 

 embryo of fifteen segments shows that the anterior cardinal vein 

 consists of three portions, namely, a long slender vessel, lying at 

 the side of the neural tube and between it and the unsegmented 

 mesoderm of the head, the vena capitis medialis; a short transverse 

 portion, which I find lies between the first and second somites; 

 and finally an irregular trunk which passes obliquely laterally 

 and backward upon the upper surface of the j^arietal plate to a 

 point opposite the third somite, where it opens into the common 

 cardinal vein or duct of Cuvier. The latter is a minute verti- 

 cal vessel which passes through the lateral plate in the mesocar- 

 dium later ale (Koelliker) and joins the vitelline vein. The first 

 indication of the anterior cardinal vein is seen in an embryo of 

 six segments (H.E.C. no. 639). The aortae are just established 

 but are still small and irregular vessels. Each aorta, however, 

 gives off into the first intersegmental cleft a branch of nearly its 

 own size which extends dorso-medially to the side of the neural 

 tube. The aorta and its branches are connected with a delicate 

 cellular network which extends between the somites, between them 

 and the neural tube, and between the medial portion of the pari- 

 etal plate and the ectoderm. The cells of this network can often 

 be distinguished from the ordinary mesenchymal cells by slight 

 differences in the intensity of the stain. They are shown by their 

 connection with the aorta and by their subsequent history to be 

 vascular cells. Two strands of these vascular cells are of par- 

 ticular and immediate interest; one of these extends from the aorta 

 upward through the second intersegmental fissure where it con- 

 nects with the second strand which extends from the termination 

 of the aortic branch in the first fissure, forward between the neural 



