40 



THE STUDY OF CHICK EMBRYOS 



The lips of the neural folds have met throughout the cranial two-thirds of the 

 embryo but have not fused. The neur al tube, form edjtlms_by--th£-t:lQsing of the 

 e ctodermal Jolds^js_02e n at either end at_^neuropores. Cep halad, th e neural 

 tube has begun to expand to form the brain vesicles. Of these only the Jore±rain 

 is prominent, ^iid from itlhe_o^U^vesicles_axeJmddmg out laterally. The 

 paraxial mesoderm is divided byjransverse furrows i nto seven pairs of b lock-hke 

 primitiv e segments. Caudally, between the segments and the primitive streak, 

 therelTundifEerentiated mesoderm, but new pairs of segments will develop in this 

 region. Looking through the open neural tube {rhomboidal sinus), one may see 

 in the midline the noiochord extending from the primitive node cephalad until it 

 is lost beneath the neural tube in the region of the primitive segments. The 

 primitive streak is still prominent at the posterior end of the area pellucida, forming 

 about one-fourth the length of the embryo. Transverse sections through the 

 primitive streak and open neural groove show approximately the same conditions 

 as in the twenty-hour embryo (Figs. 30 and 31). 



A Transverse Section through the Fifth Primitive Segment (Fig. 34) is characterized 

 by the differentiation of the mesoderm, the approximation of the neural folds and the presence 

 of two vessels, the descending aorta, on each side between the mesodermal segments and the 

 entoderm. The neural folds are thick and the ectoderm is thickened over the embryo. The 



Neural fold . 



Ectoderm 



Somatic mesoderm 



Neural groove 



Mesodermal segment 



Ccelom 



Splanchnic m esoder m 



Descending aorta Entoderm 



Fig. 34. — Transverse section through the fifth pair of mesodermal segments of a twenty-five-hour chick 



embryo. X 90. 



notochord is a sharply defined oval mass of cells. The mesodermal segments are somewhat 

 triangular in outline and connected by the intermediate cell mass, or nephrotome, with the 

 lateral mesoderm. This is partially divided by irregular flattened spaces into two layers, the 

 dorsal of which is the somatic, the ventral the splanchnic layer of mesoderm. Later, the 

 spaces unite on either side to form the ccdom or primitive body cavity. 



Transverse Section Caudal to the Fovea Cardiaca (Fig. 35) . — The section is charac- 

 terized: (1) by the closing together of the neural folds to form the neural tube; (2) by the 

 dorsal and lateral folding of the entoderm, which, a few sections nearer the head end, forms 

 the fore-gut or pharynx; (3) by the presence of the vitelline veins laterally between the ento- 

 derm and mesothehum; (4) by the wide separation of the somatic and splanchnic mesoderm 

 and the consequent increase in the size of the ccelom. In this region the ccelom later sur- 

 rounds the heart and forms the pleuro-pericardial cavity. 



