CHICK, 44 TO 48 hours' INCUBATION 21 



(4) Through the Primitive Streak 



Is the notochord present? Are the dorsal aortae 

 present? In the median portion of the body the germ 

 layers cannot be distinguished. 



Draw the section, label the partSj and color the germ 

 layers. Color black the portion in which the germ layers 

 cannot be distinguished. 



c. Reconstructions 



(i) Circulatory System 



In your sections of the thirty-six-hour chick find the 

 most anterior one which contains the blood vessels. 

 Trace these blood vessels through the remainder of the 

 sections several times. Beginning at the anterior end, 

 sketch in outline all the blood vessels in ever>' third 

 section. iXIake all your sketches on the same scale and 

 keep them in regular order. Think of these sections 

 placed one upon the top of the other in regular order. 



From your sketches make a diagram of the circulatory 

 system and label all the parts. 



3. Embryo with from Twenty-four to Twenty-nine Meso- 

 blastic Somites (Forty-four to Forty-eight Hours' In- 

 cubation) 



a. Whole Motint 



It will be observed that a number of striking changes 

 have taken place. These are most noticeable in the 

 head region. In the region of the mid-brain the head 

 has become bent so that the fore-brain and a portion of 



