to either side. As the germinal disc extends posteriorly and 

 involution of mesoderm continues, a primitive node is 

 formed above the blastopore at which entoderm and meso- 

 derm are delaminated; the entoderm is now continuous be- 

 low the notochord and the bilateral plates of mesoderm. 



As the embryo lengthens, the blastodisc also extends mar- 

 ginally at a somewhat greater rate, so that the blastopore 

 comes to lie at the apex of a notch. Meantime the head fold, 

 bearing with it the anterior ends of the neural folds and 

 neural groove, is rising up from the blastodisc. The first 

 evidences of the circulatory system, the blood islets, appear 

 in the mesoderm involuted (and in part formed by ingres- 

 sion) at the germ ring. The islets join and form the vascular 

 network of the developing yolk sac with inner and outer 

 sinuses. 



The yolk sac expands in all directions as the embryonic 

 body is constricted from the blastoderm and is added to 

 posteriorly; eventually, the tail fold is differentiated and ex- 

 tends backward above the blastopore. The neural groove of 

 the tail leads into the blastopore, which becomes separated 

 into an embryonic and vitelline blastopores by the meeting 

 of the edges of the yolk sac. At this time the caudal vein is 

 supplying blood to a terminal sinus around the outer mar- 

 gin of the germ ring, while the ring sinus, marking the inner 

 margin of the germ ring or the mesoderm-free area under the 

 head, becomes supplied with arterial blood. The embryonic 

 blastopore gradually closes as the yolk mass is overgrown 

 by the yolk sac. The gut and neurocoel are connected by 

 a neurenteric canal. This connection is soon lost. The 

 mouth is formed from a stomodeal invagination. The anus 

 is a secondary formation involving a proctodeal invagina- 

 tion. 



Te/eosf The development of the telolecithal egg of the 

 teleost follows a pattern similar to that in the shark (Figure 

 7-11). The germinal disc undergoes cleavage producing a 

 cellular mass which is united with the yolk through the 

 periblast. The periblast at any particular time appears to 

 be a syncytium, but it can be assumed that cells are differ- 

 entiated from it and are added to the blastodisc, or, later, 

 the yolk sac. The periblast extends outward more rapidly 

 than the disc and comes to enclose the yolk before the yolk 

 sac. 



The central area of the blastodisc of the teleost is raised 

 somewhat above the yolk mass and forms a segmentation 

 cavity. As the blastodisc expands, its margins fold under to 

 form a germ ring whose posterior margin is raised as the 

 blastopore. The two layers in the germ ring are the epiblast 

 and the hypoblast. The latter at the blastopore delaminates 

 into entoderm and mesoderm. With concrescence, involution, 

 and ingression, the mesodermal cells of the epiblast produce 

 a primitive node; the involuted cells then move forward 

 above the rapidly expanding sheet of entoderm. Entoderm, 

 notochord, and bilateral plates of mesoderm are delami- 

 nated at the primitive node. In some fishes the notochord 



appears to separate from the "chordoentodermal" roof of 

 the archenteron some distance from the node. 



The neural plate develops as a solid strand of involuted 

 cells similar to that observed in the lamprey. .'\s the embry- 

 onic body is differentiating anteriorly (head fold, brain, 

 and sensory organ development), the process of gastrulation 

 continues at the primitive node. Eventually, gastrulation is 

 completed and the tail fold produced with its contained 

 hindgut. The blastopore finally closes beneath the tail fold 

 in the position of the anus — separate embryonic and vitel- 

 line blastopores are lacking. 



Like the selachian, the teleost has large pellucid areas to 

 either side of the body and a mesoderm-free area in front of 

 and below the head (proamnion). The inner ring sinus or 

 vitelline veins of the teleost are venous rather than arterial 

 as in the shark. 



In the more primitive types of actinopterygian fishes, the 

 eggs do not develop in the extreme meroblastic style (Fig- 

 ure 7-12). A distinct blastodisc is not formed, rather the 

 cleavage planes are superficial and extend gradually down 

 around the egg as the animal end continues to undergo divi- 

 sions. The first three cleavages are vertical. A horizontal 

 cleavage as such is not distinguishable. Gastrulation resem- 

 bles that of the frog, but the yolk is enclosed in a syncytial 

 periblast. A part of the yolk is never intracellular. 



In vertebrates the liver is a diverticulum from the fore- 

 gut. It usually lies in front of the yolk stalk, which opens 

 into the midgut. In the case of the sturgeon, however, the 

 yolk mass lies in what might be identified as the stomach, 

 a part of the foregut, as well as in the mid and hind guts. 

 The liver is a diverticulum of the gut wall behind and be- 

 low the yolk mass (Figure 9-17). This is the one exception 

 observed in the actinopterygian fishes. 



Dipnoan The cleavages of the choanate fish, Lepidosiren, 

 resemble those of the sturgeon or the holostean (Figure 

 7-13). The first three planes are vertical and limited to the 

 animal pole. The superficial cleavages gradually extend 

 around to the vegetal end and eventually the entire egg is 

 divided into cells. The relationship between the yolk sac 

 and the gut is not generally described, but the yolk mass 

 appears to lie in the cells of the gut wall just as in the frog. 

 The liver diverticulum extends into the anterior part to this 

 yolk mass. The egg has a limited amount of yolk and the 

 larva is quite small when fully formed. 



Myxinid The telolecithal egg of the myxinid is elongate 

 ovate in shape (Figure 7-14). It is shelled and has a series 

 of hooked filaments at either end. With fertilization, discoi- 

 dal cleavage takes place at the animal-pole end of the egg. 

 The embryo differentiates at the posterior margin of the 

 blastodisc as in the teleost or shark. The blastopore closes 

 at the site of the anus. Whether the blastopore forms the 

 anus, as in the case of the lamprey, is not known. 



The heart is formed anterior to the head. A proamniotic 



CLEAVAGE, GASTRULATION, ORGANOGENESIS 



205 



