20 



MORPHOGENESIS 



the liver, the pancreas and the urinary bladder; and, with the exception of the bladder, each 

 of these becomes completely invested by primitive peritoneum. In the case of the liver this 

 original condition is practically retained, but the investment of the pancreas later becomes a 

 partial one on account of the modifications which ensue in the mesentery. The bladder has 

 only a portion of its surface in contact with the peritoneum, but the investment of the lungs 

 remains complete, each lung, indeed, appropriating to itself the entire visceral layer of its half 

 of the thorax, with the exception of a small ventral portion which forms the investment of the 

 heart. Furthermore, the cavities which surround each of the three organs named, the two lungs 

 and the heart, become completely separated from one another; and since each investment con- 

 sists of a visceral and a parietal layer, each of the organs is enclosed within a double-walled 

 sac, which in the case of each lung forms its pleura, while that of the heart is known as the 

 pericardium. The spaces which occur within the thorax between the pleurae of the two sides 

 are known as the mediastina, which include the heart, oesophagus, etc. (fig 21). 



Tn addition to the viscera mentioned there are some organs, such as the spleen and genito- 

 urinary organs, which are developments of the mesoderm, the spleen arising in the mesentery 

 which passes to the stomach and the genito-urinary organs primarily from the intermediate 

 cell mass. The morphogeny of these structures and also of the vascular system, nervous system, 

 and sense organs will be considered later in connection with their structure. 



Fig. 25. 



-Diagram op a Cross Section of the Embryonic Body and Limb. 



after KoUman.) 



(McMurrich, 



Dorsal division of 

 musculature 



Spinal cord 



Notochord 



Intestine 



Ventral division of 

 musculature 



Dorsal (posterior) division of 

 spinal nerve 



Ventral (anterior) division of 

 spinal nerve 



Dorsal limb mus- 

 culature 



Axial blastema and 

 skeleton of limb 



Peritoneal cavity 



Recession of the diaphragm and heart. — ^In the early stages of development 

 the heart is situated far forward, in what will eventually be the pharyngeal 

 region (figs. 12, 17). Just behind (caudal to) the heart, between it and the yolk- 

 sac, is a plate of connective tissue, the septum transversum, which serves for the 

 passage of large veins from the body wall to the heart (figs. 17, 23). This septum 

 together with certain accessory structures eventually gives rise to the diaphragm, 

 which becomes a complete partition separating the thoracic and abdominal por- 

 tions of the body cavity. 



The diaphragm and heart are therefore originally situated far above (cranial to) their 

 final position and recede in the course of development, producing an elongation of the vessels 

 and nerves a.ssociated with them and forcing downward such organs as the stomach and liver 

 (fig. 24). The chief factor in this displacement is probably the ventral head flexion and the 

 precocious growth and expansion of the organs in the head region. The effects of this recession 

 are especially noticeable in the nerves, tliese i)assing to the various organs concerned arising 

 from a much higlier level than that occupied by the organs. The nerve to the diaphragm, for 

 instance, comes from the fourth cervical .segment, those passing to the cardiac and pulmonary 

 plexuses from the cervical region, and those to the plexus in relation with the stomach, liver 

 and adjacent organs from the thoracic region. The blood-vessels, however, may shift their 

 origins from the main trunks by successive anastomotic roots, so that in general they keep pace 

 with the viscera in the migration caudalward. 



The limbs. — Each limb at its first appearance (fig. 22) is a flat, plate-like 

 outgrowth from the side of th(! l)ody, and consists of an axial mass (blastema) of 

 mesodermic ti.ssue from which tin; liml) skeleton will tlevelop, and, surrounding 

 this, a layer, also of mesodermic tissue, from which the muscles and blood-vessels 

 will arise. It is as yet uncertain whether the muscle blastema is derived from the 

 myotomes (as in lower vertebrates) or whether -it develops from the mesenchyme. 



