Primitive Streak and Dorsal Notockordal Opening. 17 



overgrow the e ntoderm ic streak, o r primitive plate, and when the 

 ectoderm has approached the axial line of the streak it fuses with 

 the " Randfeld " of the primitive plate, and the " Mittelfeld " is 

 soon overgrown; thus the qjitQiiej:mic primitive plate is covered 

 by ectoderm, and the primitive groove is formed. 



In discussing the question whether or no my material affords 

 support to Will's observations and theories in regard to the ento- 

 dermic streak and the primitive groove, I will take up the con- 

 sideration of my embryos in two groups. In the first group I will 

 consider embryos whose open blastopore is either a transverse slit 

 or one whos^ lateral arms bend forward. Under the second group 

 I will consider only those shields whose blastoporic crescentic 

 opening has its arms directed backward. 



a. Chelydra serpentina. — I will describe first a specimen in 

 which two embryos are formed on the same germ disk, the second 

 case of this sort that has occurred in my small collection of 

 twenty-six embryos. As in the case next to be described, so also 

 in the present one, it will be observed that the two individuals 

 (Plate II. Fig. 5) are not in the same stage of development. The 

 younger individual, which we will designate as a, is distinguished 

 from the older individual (/3) primarily by the lack of the opening 

 of the blastoporic or notochordal canal on the dorsal surface. 

 The blastoporic opening in individual /3, which is a narrow elon- 

 gated slit whose concavity is directed forward, separates its me- 

 dullary groove from its primitive streak. Individual a however 

 shows, at least on surface view, no such separation into primitive 

 streak and medullary groove. If both exist, they are continuous. 

 Embryo a, which lies exposed throughout its whole length, is, as 

 has been previously stated, somewhat younger or more retarded 

 in development than the one (/3} which it partly conceals. 



Rauber (^TT, p. 79) has found unequal rate of development to 

 exist in the case of double embryos of bony fishes. Burckhardt 

 ('88, p. 431), however, has observed the existence of two chick 

 embryos on the same disk which are in the same stage of develop- 

 ment. These chick embryos possess a primitive groove in both 

 head and tail regions. 



At the point of fusion or contact between the two turtle em- 

 bryos (Fig. S) an irregularity — a kind of folding — is noticeable 

 on the ectodermal surface. 



In order to determine the cause of this surface irregularity and 



