Abnormal Development of Toad Ova 21 



terminates in a mass of cells filled with yolk and presenting no well 

 marked lumen. Toward the posterior end of the trunk a lumen 

 appears in the dorsal region of this mass. This lumen may be 

 followed to the anus. 



The sinusoidal circulation of the liver is well established. The 

 bile duct is of some length and opens into the intestine slightly 

 to the left of the mid-line of the body. The anlage of the gall 

 bladder may be s^en below the bile duct. 



The anlage of the pancreas is marked by a deeply staining mass 

 of cells posterior to the anlage of the liver on the left side of the 

 body. The ducts at this stage are not clearly marked in this 

 embryo. 



Heart and Blood Vessels — The heart is an S-shaped tube, the 

 lumen of which is filled with blood. There are no well-marked 

 trabeculae in the ventricle. The main arteries and veins are filled 

 with blood and may be readily followed. In general they seem 

 to correspond with the diagrams of the blood vessels for the 

 recently hatched tadpole given by Marshall (Embroylogy, p. 170, 

 Figs. 'JJ and 78). 



Reproductive Organs — The coiled tubules of the head kidney 

 are surrounded by a vascular plexus. Distally the two Wolfian 

 ducts have a common opening into the cloaca. They are patent 

 throughout. 



Skeleton — The notochord and a loose mesenchyme formed of 

 anastomosing cells constitute the skeletal tissues of this embryo. 



Muscles — In the region of the head the muscles anlages are 

 marked by dense masses of tissue, in some of which specific differen- 

 tiation has begun. In the region of the spinal cord the myo- 

 tomes form a well differentiated segmental musculature on each 

 side of the chorda dorsalis, the spinal cord and the spinal ganglia. 



Control Larva of the Age of the Experiment Larvae 



The control larvae, Plate I, at the period when most of the 

 experiment larvae were preserved for examination were about 

 at the stage of development described by Marshall for the 12 mm. 

 tadpole of the frog. The organs may be briefly described as 

 follows : 



