PLATE 4 



EXPLANATION OF FIGURES 



19 to 31 Origin of plastids. Cytological characteristics of erythrocytes. 



19 Experiment 3; twenty-one-hour homoplastic culture of blood from a 28 mm. 

 embryo in ])lasma from a 52 mm. embryo. A constriction rather irregular in char- 

 acter, showing the origin of three small plastids (stage/), one of which while under 

 observation was seen to assume a cup shape (stages g-n). In this instance the 

 plastid was undergoing a slow rotation, presenting various views as shown in 

 stages m and //. 



20 Experiment 9 ; nine teen-hour homoplastic culture of the blood from a 32 mm. 

 embryo in plasma from a 49 mm. embryo. Shows a somewhat imperfect case of 

 constriction in which the cytoplasm between the nucleus and the forming plastid 

 became drawn out in a slender thread before the plastid became detached. 



21 Erythroblast from heart of 35 mm. embryo. Beginning at the left, a, b, c, 

 and d are successive aspects presented during a single rotation of the cell as it was 

 mechanically turned over while under observation, showing the tendency of the 

 nuclear pole to swing undermost in response to gravitation. The profile view c 

 also shows the biconcave disc form. 



22 Ei'vthroblast from 28 mm. embryo, after having been kept two days in 

 Ringer's solution. Shows an unequal response of the nuclear and cytoplasmic 

 poles to osmotic changes; h is the profile view of n. 



23 and 24 Erythroblasts which, although containing two nuclei, have already 

 assumed cup and flattened disc forms, respectively; from a 25 mm. embryo. In 

 each case b is a profile view of a. 



25 Face n and profile b views of an erythroblast showing a constriction in the 

 region between the cytoplasmic and nuclear poles. From a culture of the blood 

 of a 32 mm. embrj'o in the plasma of a 65 mm. embryo. Experiment 60. 



26 Face o and profile b views of a small plastid showing biconcave disc form; 

 25 mm. embryo. 



27 and 28 Small and medium sized |)lastids which by mechanical rotation were 

 demonstrated to be sjiherical in shai)e; 28 mm. embryo. 



29 to 31 Medium sized and large plastids showing cup and disc shapes. Each 

 figure represents successive views of a mechanically rotated cori)uscle, cf. fig. 19. 



202 



