19.22] Phases in the Development of Ciirysemys Cinerea 69 



They show essentially the same features as are figured by Mitsukuri 

 and Ishikawa. 



Similar conditions are not found in Chnjsemys cinerea, since the 

 yolk is plainly covered with ectoderm for some distance down into 

 the blastoporal canal (see fig. 29). This is lirought about in the fol- 

 lowing manner : At the posterior end of the primitive plate, before 

 the beginning of invagination, there is proliferated a group of cells 

 of endodermal origin. These are covered over with a layer of ecto- 

 derm. When invagination begins, the ectoderm forms a cup which 

 seems to sink into these lower cells. Ultimately the ectodermal cells 

 break and the part forming the anterior wall of the blastoporal cavity 

 turns cephalad, forming a layer of cells under the ectoderm. In 

 Chrysemys cinerea this rupture of the ectoderm takes place at the 

 bottom of the cup when it has reached its greatest depth before 

 turning cephalad. This leaves a part of the endodermal cells of 

 the primitive knob covered with ectoderm, as indicated above. On 

 the other hand, Mitsukuri (1896) found in the forms which he studied, 

 {Trionyx Japonicus and others), that the primitive knob was never 

 completely covered with ectoderm but that when the ectoderm had 

 spread OA^r about one-half the outer surface it began to invaginate, 

 leaving the latter half of the primitive knob uncovered. To this un- 

 covered part he applied the term yolk plug. Earlier in this paper 

 such a condition has been shown to exist in Chelydra serpentina. This 

 was the basis upon which Mitsukuri constructed his theory as to the 

 homology of this structure in EJasmohranchii, Chelonia, Amphihia 

 and Mammalia. He states that no such structure (i. e., yolk plug) 

 has been reported for birds, "probably due to the fact that nobody 

 has looked for it." He finds a closer analogy between Elasmohranchii 

 and Chelonia than between Amphibia and Chelonia. In the exami- 

 nation of several species he found that the more specialized the 

 Chelonian studied, the less distinct was the yolk plug. If this be 

 true, the non-occurrence of yolk plugs in Chrysemys cinerea means 

 that this species is farther removed from the ancestral type than 

 Trionyx japonicus and Chelydra serpentina. Other facts concern- 

 ing these two forms lead to the same conclusion. 



Soon after invagination begins there is a thickening of the ecto- 

 derm in the median longitudinal line. This represents the primitive 

 plate. Previous to this thickening the head fold becomes visible at 

 the anterior end of the disc, but it is not nearly so marked as is 



