82 THE EARLY DEVELOPMENT OF MAMMALS. 



ment, when first formed, appears more or less nearly square in surface views, and 

 triangular in cross-sections. As the medullary canal grows, so does the secondary 

 segment, and it becomes, therefore, somewhat elongated in its dorso-ventral 

 diameter. After this change in its shape we can distinguish in transverse sec- 

 tions of an embryo (Fig. 28) the outer wall, which lies under the ectoderm, and 

 an inner wall, which lies toward the medullary canal and notochord. In the 

 further history of the secondary segment we can distinguish the following steps : 

 First, the production of the myotome, with the accompanying transformation of 

 a portion of the cells of the inner wall of the segment into the mesenchyma ; 

 next, the production of the true muscle plate; third, the breaking-up of the outer 

 wall of the myotome. These portions are sufficiently described in the practical 

 part, Chapter V. 



The Splanchnocele. The splanchnocele makes its first appearance in the 

 parietal zone of the mesoderm in the manner above described. It rapidly in- 

 creases in size, so that a considerable space separates the somatic from the 

 splanchnic mesoderm, as shown in Fig. 165 and Fig. 162. When it first appears, 

 it is a narrow fissure. It rapidly widens, extends toward the axis until it almost 

 reaches the primitive segments, and also spreads out laterally into the so-called 

 extra-embryonic region. As above stated, the rate and extent of its extra- 

 embryonic development vary greatly in different mammals. It develops earlier 

 and acquires a greater distention at first in the future cervical region, where it 

 produces the amnio-cardiac vesicles, in the median portion of whose united cavi- 

 ties the heart is lodged. The splanchnocele of the body proper appears after the 

 primitive segments, and its expansion takes place at first only in the part of the 

 mesoderm next to the primitive segments. Everywhere as the splanchnocele 

 develops the mesodermal cells about it assume gradually more and more dis- 

 tinctly an epithelial character, so that it soon becomes proper to speak of the 

 mesothelium or boundary epithelial wall of the coelom. 



The splanchnocele is also designated by several other names, and is some- 

 times called simply the body-cavity or somatic cavity. Others term it the ventral 

 ccelom. By English embryologists it is usually called the pleuro- peritoneal space. 

 Its future subdivisions become early indicated by a transverse ridge of tissue 

 which is known as the septum transversum. This septum is situated at the poste- 

 rior end of the heart, and is developed to allow the great veins to have access to 

 the heart itself. It is the anlage of the future diaphragm. It separates the 

 coelom around the heart from that of the abdomen. It is a product of the splanch- 

 nopleure, so that it rises up on the ventral side of the coelom. We have, as 

 soon as this septum is present, the pericardial cavity on its cephalic side, the 

 abdominal cavity on its caudal, and a small pleural cavity on its dorsal side. 



