separate anteriorly. These are formed from materials of the 

 roof of the archenteron. The mesoderm forms solid bands to 

 either side of the medullary plate as it sinks down, and the 

 notochord lies between these mesodermal ridges. In the 

 region of the prechordal plate of mesoderm, three pairs of 

 preotic coelomic pouches, lying anterior to the otic cap- 

 sule, are indicated. These pouches, and also the notochord, 

 are formed very much as they are in Amphioxus. The 

 lateral walls of the archenteron now move medially below 

 the notochord and pouches to form the entodermal roof of 

 the pharyn.\. More posteriorly, the mesoderm forms outside 

 of the presumptive gut, without any evident relationship to 

 the lumen of the archenteron (evolution of cells from gut 

 wall). 



The floor of the archenteron is thick with cells which are 

 large and contain much yolk. Very early in development, a 

 diverticulum of the archenteron extends downward into the 

 mass of yolk cells, forming the aniage of the liver. The cavity 

 of the neural tube begins to form anteriorly and to extend 

 posteriorly. Since a neurenteric canal is lacking, the neural 

 cavity is never in connection with that of the gut. 



The liver diverticulum branches and liver cells appear in 

 conjunction with these branches. The heart lies ventral to 

 the pharynx, anterior to the liver diverticulum. A stomo- 

 deum appears anteriorly; the blastopore remains open as 

 the anus. The membrane between stomodeum and pharynx 

 ruptures to form the mouth, part of it remaining as the 

 velum. The midline invagination for the olfactory capsule 

 and the hypophyseal sac lie outside the mouth (Figure 

 12-1). As the buccal funnel develops, the common opening 

 for these pouches is displaced to the top of the head. 



Eight pairs of pharyngeal pouches develop, and all open 

 to the exterior except the first, or spiracular, pouch. The 

 hyoid arch has anterior and posterior hemibranchs, but later 

 loses the anterior one. An elaborate "thyroid" gland forms 

 from the subpharyngeal gland, or endostyle. 



In its development, the lamprey resembles Amphioxus in 

 that several pairs of anterior mesodermal pouches are 

 formed. The mesoderm and notochord have a close rela- 

 tionship to the roof of the archenteron, for the entoderm 

 moves medially below this tissue to form the roof of the 

 pharynx. The lamprey larva is similar to Amphioxus in its 

 way of life and filter feeding mechanism. They differ in the 

 origin of the neural tube and the lack of a neurenteric canal 

 or neuropore. 



The development of the lamprey is quite different from 

 that of the myxinid (Figure 7-14). The latter has an 

 extreme telolecithal egg with a germinal disc that cleaves 

 discoidally. It will be considered after the frog and chick 

 types have been examined. 



Frog The egg of the frog is mesolecithal with a distinct 

 ventral concentration of yolk (Figure 7-1 C). The animal 

 half of the egg is covered by a thin cytoplasmic film in which 

 there is melanin (black) pigment. This pigment is assumed 



to warm the eggs when they are exposed to light and thus 

 increase their rate of development. In contrast to this, there 

 is the idea that in the lamprey light slows down the devel- 

 opment of the eggs. 



The egg is covered by a thin chorion and vitelline mem- 

 brane, although some writers recognize only the presence 

 of a vitelline membrane. When the eggs are shed, they have 

 three unexpanded jelly coats, which may be but an exten- 

 sion of the chorion. The eggs are fertilized at the time of 

 shedding; following fertilization, the jelly coats begin to ex- 

 pand and eventually are twice the diameter of the egg 

 itself 



Fertilization occurs in a band 35 to 40 degrees from the 

 central axis of the egg, and the path of the male gamete is 

 marked by its drawing a strand of pigmented cytoplasm 

 along behind it (Figure 7-7). This penetration path results 

 in movement of pigment from an area opposite the point of 

 entrance; this area of diminished pigment is identified as 

 the gray crescent. The penetration path usually, but not in 

 all amphibians, marks the path of the first cleavage plane. 

 This plane passes along the penetration path through the 

 animal pole and the middle of the gray crescent; it sepa- 

 rates the egg into right and left halves. 



The second cleavage plane is at right angles to the first 

 and separates the animal into dorsal and ventral halves. The 

 third cleavage is usually an equatorial one dividing the four 

 cells into animal and vegetal quadrants. The division of the 

 four cells are not synchronous or necessarily in one plane. 

 The subsequent cleavages are more asynchronous. 



At the eight-cell stage there is already a small blastocoel, 

 but this becomes more distinct with further divisions. The 

 blastula has several layers of small cells at the animal pole, 

 and many layers of larger cells at the vegetal pole. The 

 blastopore begins to form as the ventral margin of the gray 

 crescent. 



The process of gastrulation involves invagination, involu- 

 tion, and epiboly. The more actively dividing cells of the 

 animal pole, particularly those at the donsal lip, are respon- 

 sible for these movements. Many of the internal structures 

 are traceable to surface areas of the blastula; some of these 

 are indicated in Figure 7-8. The cells move from these areas 

 to the dorsal lip of the blastopore, enter the blastopore, and 

 then move away from that opening, within the gastrula, to 

 their final position. The moving of cells into the gastrula 

 through the blastopore (involution) produces a roof for the 

 archenteron of animal pole cells or mesentodcrm. The layer 

 of cells next to the archenteron is entodermal; the layers 

 above are mesodermal and give rise to a medial notochord 

 and lateral plates of mesoderm. 



The plates of mesoderm separate from the notochord and 

 entoderm at the anterior end of the embryo first. The noto- 

 chord begins to separate from the entoderm anteriorly and 

 this process continues in a posterior direction. In the region 

 of the dorsal lip of the blastopore, the ectoderm, mesoderm, 

 and entoderm form a common mass. 



CLEAVAGE, GASTRULATION, ORGANOGENESIS • 199 



