zona radiata (shell) 



central eel 



marginal blastocone 



ACIPENSER 



A FIRST CLEAVAGE 



B THIRD CLEAVAGE 



CLEAVAGE 



EARLY BLASTULA 



AMIA 



hypoblast cells in blostocoel 



central cells of blostodisc periblast (budding ofF ^^iJ^^^g^^I epiblast 

 hypoblast ce 



yolk 



4 CELL 



F THIRD CLEAVAGE G EARLY CLEAVAGE 



H SECTION OF BLASTULA 



neural keel prechordal plote (mesentoderm) 



ntoderm 



neural keel 



— blastopore 



mesoderm 



Kupffer's vesicle 

 yolk plug 

 mesoderm 



I 



LATE GASTRULA 



LATE YOLK PLUG STAGE 



Figure 7-12. Stages of Acipenser (A-D) and Amio (E-J). A, first cleavage; B, third cleavage fur- 

 rows; C, fourth cleavage plains cutting off central cells from marginal blastocones; D, early blostulo; 

 E, 4-cell stage of Amio; F, third cleavage furrows in Arnio; G, section of early cleavage blostodisc 

 of Am'ia; H, semidiogrommotic blostulo of Amio; I, sagittal section of yolk plug stage of Amia (late 

 gostrulo); J, neurula stage of Amia. (After Dean, 1895, 1896) 



resembling that of some of the simplest animals (Figure 

 7-1 E). 



The egg of the monotreme is shelled; albumin encloses a 

 large yolk mass on which a small germinal vesicle lies. 

 When fertilized this egg cleaves discoidally, producing a 

 mass of cells, the blastodisc. This blastodisc is connected by 

 periblast cells, forming a zone of junction, with the yolk 

 mass. 



As the blastodisc expands, it tends to thin to nearly 

 a single layer of cells; from this, by multipolar (many 

 points) ingression, an entoderm layer is formed below what 



is now the epiblast. The entoderm layer expands rapidly in 

 all directions until it underlies the entire blastodisc and en- 

 closes a yolk sac. Mesoderm is formed from the epiblast by 

 the usual movements of concrescence and ingression of 

 cells which form first a blastopore, then a primitive streak. 

 At the anterior end of the primitive streak is a pit and a 

 Hensen's node. 



A mesodermal prechordal plate is formed by thickening 

 of the epiblast and involution of a small portion of the pos- 

 terior margin of the blastodisc at the "blastopore." This 

 prechordal plate later separates into head mesoderm and 



CLEAVAGE, GASTRULATION, ORGANOGENESIS 



207 



