ABBREVIATIONS 



Ect., ectoderm Pro.du., pronephric duct, or anlagal 



G.tp., growing tip of the anlagal duct duct 



Ldm.m., limiting membrane Pron., pronephros 



A^f.c, notochord Sp.c, spinal cord, or neural tube 



PLATE 1 



EXPLANATION OF FIGURES 



1 Frontal section of a 5 mm. embryo, showing the pronephros arising from six 

 somites, the sixth participating but little in this section. The fifth curves out- 

 ward toward the anlage of the pronephric duct, a, with which it is to unite, thus 

 forming a complete tubule. The duct will arise on the lateral edge of the proneph- 

 ric body, where the nuclei are beginning to arrange themselves in line, h. At the 

 caudal end the anlage of the duct projects into the space between ectoderm and 

 somites. The extent to which it has grown may be seen by comparing with figures 

 2 and 3. 



2 The same parts as in figure 1, illustrating method of growth. Xote the nu- 

 merous mitotic figures in the pronephric anlage and one in the anlage of the duct, 

 indicating rapid growth; ectoderm a single cell thick. 



• 3 Growing tip of figure 1 under high power, to show its relation to the ecto- 

 derm. 



4 and 5 From cross sections of a very earh' embryo. They correspond to some 

 figures in Balfour's Monograph and show a very early pronephros and its origin 

 from the middle plate. 



5 Same, more enlarged, showing contact with ectoderm at a. 



6 and 7 From embryos of 4.5 and 5 mm., illustrating the manner of growth of 

 pronephros toward ectoderm, the pronephros having been pulled away in figure 

 6, leaving its impress. This separation, caused by shrinking in preparation, dem- 

 onstrates the independence of the two structures. Figure 7 illustrates the manner 

 of growth of the pronephros; the clear translucent cytoplasmic processes at a and 

 h are very characteristic. 



8 Shows some isolated cells bearing such processes 



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