REPRODUCTION AND DEVELOPMENT 53 



the mid-dorsal region between ectoderm and cndoderm. This structure 

 is the notochord, or primitive skeletal axis. What groups of animals 

 possess notochords? Coincidentally with mesoderm and notochord 

 formation the first localization of organ-system primordia begins in the 

 ectoderm on the dorsal surface. After an increase in thickness by 

 which the neural plate is distinguished from the remainder of the ecto- 

 derm, cell movements begin and produce a lifting of the edges of the 

 neural plate to form the neural folds. Examine specimens in which 

 these folds are forming. How are the right and left neural folds re- 

 lated to each other at each end of the neural plate? Find the blasto- 

 pore, which now lies at the posterior end of the area enclosed by the 

 neural folds. What is the position of the blastopore with reference to 

 the neural folds? Notice that the neural folds are farther apart at the 

 anterior end. Draw this stage from a dorsal view and make an outline 

 of the jelly still surrounding the embryo. The jelly envelopes begin to 

 disintegrate at about this time, and preserved embryos are likely to 

 become separated from them before the time of hatching. 



Exercise 56. — The Late Neural-fold Stage. 



(b) Examine an older embryo. The neural folds have become 

 higher and approached each other along the dorsal mid-line. What 

 happens to the neural folds? What becomes of the blastopore? What 

 general changes in the shape of the embryo are taking place? Under- 

 stand what organ-system is differentiated from the cells of the neural 

 plate which have now become localized. Draw from a side view, 

 orienting the embryo as you did in Exercise 54 (c). 



Exercise 57. — The Late Embryo. 



(c) Conspicuous changes are beginning to appear in the shape of 

 the embryo. Several specimens of slightly different ages are desirable, 

 since the primordia are not easily recognized. Determine first the 

 regions of the body in relation to the previous stage. The dorsal sur- 

 face is less curved than the ventral. Where is the line of fusion of the 

 neural folds? The head end is blunter than the tail end at this stage. 

 On each side between the trunk and the head there is a slightly raised 

 area, the gill plate. Anterior to this is the primordium of the sucker, 

 recognizable as an unpaired U-shaped structure when seen from ven- 

 tral view. Locate the proctodeum, or future anus, at the posterior 

 end of the embryo. The specimens should be rolled over and studied 

 from different angles. Draw one from a side view, oriented in the 

 same way as the last figure. 



(d) If living material is available the movement of the embryo 



