56 THE CIRCULATORY SYSTEM 



resence of an irregularly shaped nucleus and by a cytoplasm which possesses 

 'anules of various dimensions and staining affinities, 

 b) Lymphoid Forms. Lymphoid forms are of two types, namely, lympho- 

 f'tes and monocytes. These cells arise from lymphoblasts and monoblasts 

 :spectively (columns 1 and 2, fig. 333A). The lymphocytes are small, 

 )unded cells with a clear cytoplasm and a large nucleus. They are found 

 I all vertebrates and are abundant especially in fishes and amphibia. Large 

 jmbers are found in the lymph nodes in various parts of the body. Monocytes 

 :e similar to the lymphocytes but are much larger and have a tendency to 

 Dssess an irregularly shaped nucleus. Various hematologists hold that the 

 lonocyte is a special type of blood cell, distinct from other leukocytes and 

 " a separate developmental origin. 



D. Morphogenesis of the Circulatory System 



1. Introduction 



The major alterations of the basic arterial and venous conditions into 

 le morphology present in the adult or definitive body form of the species 

 ;cur during the larval period, or the period of transition from primitive 

 Tibryonic body form to the definitive or adult form. This fact is true not 

 [ily of the circulatory system but of all other organ systems as well (Chap. 

 1). The pronounced changes, therefore, which occur in the revamping of 

 le basic, generalized condition of the circulatory system during the larval 

 jriod should be regarded as transformation which adapts the basic embryonic 

 jndition to conditions which must be met when the developing organism 

 nerges into the environment of the adult. 



2. Transformation of the Converging Veins of the Early 



Embryonic Heart into the Major Veins which 



Enter the Adult Form of the Heart 



a. Alteration of the Primitive Converging Veins of the Heart in the 

 Shark, Squalus acanthias 



An early stage of the developing venous circulation of Squalus acanthias 

 shown in figure 334A. Only two veins are present, the primitive vitelline 

 iins. They enter the sinal rudiment of the developing heart. Before the liver 

 ibes form, the left vitelline vein develops a new venous sprout, the intestinal 

 iin, which extends caudalward along the lateral aspect of the intestine to 

 le developing cloacal area (fig. 334B). Here it forms a collar-like venous 

 ructure around the cloaca and continues back below the tail gut as the 

 mdal vein. Meanwhile, the anterior, posterior, and common cardinal veins 

 sgin their development, and the liver also begins to form (fig. 334C). As 

 le liver develops, two prominent liver lobes are elaborated (Scammon, '13), 

 id the vitelline veins become surrounded by the developing liver trabeculae 



