HASIC liEI'RoDrcriVI': .\M> llEUEhlTV I'llESoMKS.X .'ili 



7. Yolk-plug stage. Tlu* cresccntic j^roovc is (h-cpcbt at the 

 center and thins out toward the edges, whicli gradually ext<'nd 

 around the vegetal pole of the egg. In this way the crescent 

 becomes a circle and as the white-cell area is gradually over- 

 grown a stage is reached when only a small circle of white yolk 

 appears in the midst of the black covering. This is known as 

 the yolk plug, socalled since the yolk cells appear as a plug in the 

 blastopore. The white cells which have been enclosed by the 

 dark and other cells which have been overgrown or invaginatcd 

 now become the entoderm. 



By the time the yolk-plug stage is reached the formation of 

 the mesoderm or middle germ layer is well underway. It develops 

 from the cells around the blastopore which proliferate and push 

 their way in between the ectoderm and entoderm with the result 

 that the embryo comes into the possession of three layers of 

 cells. These are the primary germ layers, furnishing the cells 

 out of which the completed body is formed. The method 

 followed is that which may be expressed as infoldings, outfold- 

 ings, and thickenings of the layers and special changes in the 

 component cells. 



8. Neural-fold Stage. — This stage is recognized in its early 

 phases by the elongation of the embryo and by the appearance 

 of a fold on each side of a median groove extending lengthwise 

 along the future back of the individual. The edges of these 

 folds, the neural folds, later meet and fuse along the median dorsal 

 Hne, thus forming a longitudinal tube which sinks beneath the 

 ectoderm from which it is separated and forms the central nervous 

 system. The yolk plug has finally disappeared by further 

 overgrowth, leaving only a minute pit to indicate the former 

 position of the blastopore. This pit may be covered over by 

 the neural folds in this region but a short distance behind this 

 point the anus begins as an invagination of the ectoderm which 

 later meets and fuses with the archenteron to establish an open- 

 ing, the anus, between the latter and the exterior. 



The beginning of the mouth may be seen as a hollow depression 

 of the ectoderm, but it does not comnumicate with the archenteron 

 until after hatching. 



Dorso-caudad of the ventral suckers there develops an eleva- 

 tion in which two vertical grooves appear. These grooves 

 represent the first two gill slits. Later two more grooves appear, 

 one cephalad and one caudad of the first pair. At the time of 



