second polar body 

 first polar body / fertilization membrane 



second polocyte 

 anterior 



A EGG 



ventral 



posterior 



D 8 CELL 



EARLY BLASTULA 



neural plate neural fold neurenteric 



. L J "\ \ canal 



notochord^ 



F BLASTULA G GASTRULA H 



Figure 7-2. Cleavage and gostrulation as observed in annphioxus. A, fertilized egg v/tth two polar 

 bodies; B, 2-cell stage; C, 4-cell stage; D, 8-cell stage; E, late cleavage with axis of larva indicated 

 by arrow; F, blastula in sagittal section, axis indicated by arrow and ventral side down; G, gastrulo; 

 H, early neurulo. (After Conklin) 



position of blastopore 

 EARLY NEURULA 



parates the egg into right and left halves, and the second is 

 at right angles to the first. They intersect below the polar 

 body. 



The third cleavage plane, a synchronous division of each 

 of the four cells, is usually an equatorial one separating each 

 cell into top and bottom halves, but it may be a meridional 

 one. The top cells are slightly smaller than the bottom cells. 

 After this, there are vertical cleavages of each dorso-ventral 

 pair of cells followed by an essentially synchronous hori- 

 zontal cleavage of each of the resulting 16 cells. At this 

 stage, there are now 32 cells. By the 32-cell stage, cleav- 

 ages have become asynchronous, and the cells of the top 

 half are noticeably smaller than those below. 



The resulting blastula is a hollow ball of cells. The cavity 

 is the blastocoel with a wall a single layer of cells thick. As 

 the blastula develops, it flattens ventrally (at the vegetal 

 pole) and this area gradually invaginates; that is, it appears 

 to be pushed into the more rapidly dividing hemisphere of 

 the animal region. The process of gastrulation also involves 

 some outgrowth of the rims or lips of the blastopore. As the 

 process continues, the blastopore constricts to a small open- 

 ing. 



When completed, the gastrula is a two-layered structure 

 with an internal cavity, the archenteron, which opens to 

 the exterior through the small blastopore at the posterior 

 end. The polar body marking the original animal pole now 

 lies ventroanteriorly in the region of the head, A neural or 

 medullary plate diffisrentiates on the dorsal surface of the 

 gastrula and begins to sink down, starting from the dorsal 



blastopore lip (Figure 7-3 A), It is overgrown by the mar- 

 gins of this plate, beginning with the ventral blastopore lip. 

 The crescent-shaped margin of ectoderm moves medially, 

 while its posterior margin moves forward and dorsally (Fig- 

 ure 7-2 H), Concurrently, the margin extends more ante- 

 riorly as the medullary plate begins to sink in this region. 

 The larva is now called a neurula. 



The neural plate thus differentiates from back to front and 

 is overgrown by ectoderm from back to front. The plate, now 

 covered by ectoderm, rolls up laterally to form a tube from 

 back to front. As a result of this action, the blastopore is 

 closed, but the archenteron is connected with the exterior 

 through the neural canal and the anterior neuropore. The 

 cavity of the neural tube is connected with the gut by the 

 neurenteric canal (see Figure 7-7 K). 



The entoderm is composed of somewhat larger cells than 

 the ectoderm since it is derived from the large cells of the 

 vegetal half of the blastula. The dorsal part of this entoderm 

 is derived by involution of cells from the outside. The dorsal 

 wall of the archenteron underlying the neural plate and tube 

 now begins to form bilateral outpocketings or pouches at 

 the anterior end of the neurula (Figure 7-3 B). The first of 

 these pouches lies somewhat behind the anterior end and 

 new pairs arise serially behind it (Figure 7-3 D). Between 

 these pairs of pouches, the roof of the archenteron forms the 

 notochord, which arises as an evaginated groove; the edges 

 of the entoderm move towards the midline below this 

 grooved plate. The notochord and coelomic pouches are of 

 apparent entodermal origin in this group. 



CLEAVAGE, GASTRULATION, ORGANOGENESIS • 195 



