78 J. Thos. Patterson. 



uniting along the middle line of the future embryo. This process 

 of "concrescence" is operative in all cases, even though there is 

 no perceptible streak in the majority of blastoderms. The question 

 of concrescence will be considered in connection with the section 

 on experimental studies. 



B. Study of Sections. 

 a. Pregastrular Stages. 



In the study of sections, it will suffice to begin by describing 

 a blastodisc in what I shall term a late cleavage stage. A median 

 longitudinal section (Fig. 1, PI. I) shows that the blastodisc is 

 thinner directly above the "Nucleus of Pander" than in other regions, 

 except at the margin where it may be but one cell thick (Fig. 1, a). 

 This thin marginal condition corresponds to the less opaque marginal 

 zone seen in surface views from the twentieth to the twenty-eighth 

 hours, and is brought about, as Aliss Blount has shown, by the manner 

 in which the blastodisc is increasing in diameter. External to the 

 margin are periblastic nuclei about which cells are formed and 

 added to the edge of the disc (anterior end of Fig. 1). This process 

 may continue to such an extent that a row of several cells will be 

 seen in section. This is not always the case, for periblastic nuclei 

 are also present in the yolk lying directly beneath the thin margin ; 

 and about these nuclei, cells are organized and added upward to the 

 disc, so that the margin may become more than a cell thick (posterior 

 end of Fig. 1). Directly above the Nucleus of Pander, between the 

 white yolk and the deeper cells of the disc, is the fissure-like seg- 

 mentation cavity (sc), and between the edge of this cavity and the 

 margin of the disc is a zone, in which the cells are open below to 

 the white yolk. This region is more or less of a syncytium, in which 

 cell boundaries are either wanting or very indistinct. It exists 

 around the entire margin of the disc, and constitutes the zone of 

 junction.^^ 



^"In my preliminary paper I used the term germ wall to designate this zone, 

 but for the sake of unity it has seemed advisable to employ the term zone of 

 junction instead. There is no objection to using this term to designate ';he 

 entire zone at this stage, at least so long as one bears in mind the fact that 

 the inner part of this zone is potential germ wall. 



