THE EMBRYONIC CCELOM. 87 



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 (Figs. 45 A and B, Coe). 

 It rapidly increases in size, so that a considerable space separates the somatic from 

 the splanchnic mesoderm, as shown in figures 160 and 163. When it first ap- 

 pears, 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 in birds 

 earlier and acquires distention first in the future cervical region, where it produces 

 the amnio-cardiac vesicles (Fig. 136, A.c.v), in the median portion of whose 

 united cavities 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 tfre primitive segments. Everywhere as the splanchnocele 

 develops the mesodermal cells about it assume gradually more and more distinctly 

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

 or boundary epithelial wall of the ccelom. 



The splanchnocele is also designated by several other names, and is sometimes 

 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 posterior 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 ccelom around the heart 

 from that of the abdomen. It is a product of the splanchnopleure, so that 

 it arises upon the ventral side of the ccelom. We have, as soon as this septum is 

 present, the pericardial cavity on its cephalic side, the abdominal cavity on its cau- 

 dal, and a small pleural cavity on its dorsal side. 



The Ccelom of the Head. No adequate investigation of the early stages of the 

 mesoderm in the head of amniota has yet been made. We know, however, that 

 in the lower vertebrates there appear at least three distinct cavities resembling por- 

 tions of the true ccelom and bounded by epithelial cells, similar to the mesothelium 

 in character. These cavities are generally regarded as portions of the true ccelom, 

 and by many writers have been interpreted as true primitive segments. But this 

 interpretation is not yet beyond doubt. The largest of these is called the mandibu- 

 lar cavity, because it has a prolongation which extends into the mandible of the 

 young embryo. In front of it is the first or premandibular cavity, which is much 

 smaller, and behind it is the third or hyoid cavity, which is intermediate in size 

 between the first and second (Fig. 48). The head-cavities are best known in the 

 elasmobranchs. They have also been found clearly developed in reptiles and cer- 

 tain birds. In mammals no actual cavities have been recorded. There are found 



