shrunk from his task
See Plate 11, depicting Los as exhausted, sickened, and horrified beside the new formed Urizen.
His fires beheld
The fires are personified here (Stevenson 259n), suggesting that they are extensions of Los's eternal nature or are themselves Eternals.
deadly sleep
Urizen is no longer awake to eternal life and the transformations of Eternity, which "Roll like a sea around him" (13.30), unperceived by his shrunken senses.
him
The referent is ambiguous. It could be either Urizen or Los.
Like a dream was obliterated
For Urizen or for Los, his life in Eternity is like a dream which he forgets when he awakens; however, here he forgets it when he enters a "deadly sleep" (13.27).
smote / With a stroke
Los suffers a stroke, which impairs him. That he is "smote / With a stroke" suggests a hammer blow such as he has used to bind Urizen. As a result of this stoke, his instruments are silent and "his prophetic voice / Seiz'd" (13.38-39) like a machine that is frozen and is unable to function. This stroke is an indication of Los's fall.
clos'd
Los and Urizen draw together, grasping one another as wrestlers do when they "close"; they are also closed together in a globe within a void, Urizen's world.
frozen / Into horrible forms of deformity
Not only is Urizen trapped into a single immutable and, from the point of view of the Eternals, limited and thus deformed body, but also Los is trapped in such a body.
his fires to decay
Los's essence as an Eternal, his energy (symbolized by fire), is diminished as he falls into Urizen's world.
anxious desire
Los looks back toward Eternity with both the fear that he has fallen from it and the wish to rejoin it. Rather than the plenum of Eternity, filled with Eternals and their creations, he sees only empty space, the void of Ulro.
pity divides the soul
As Los looks on Urizen with Pity, his soul begins to divide into two, Los and Enitharmon. The design on Plate 13 depicts a female, Pity and/or Enitharmon, dividing the heavens, represented as clouds divided by a void. Plate 14 depicts a Urizenic figure suspended between two clouds.
a round globe of blood
Plate 17 depicts the globe of blood that is divided from Los and forms Enitharmon.
now seen, now obscur'd, to the eyes / Of Eternals
In the design on Plate 15, the Eternals peer into the abyss created by the fall of Urizen and Los.
Branching out into roots
Just as Urizen's heart branched into the circulation system (11.1-7), so Enitharmon's heart branches, but into "Fibres of blood, milk and tears" (18.4), the "milk" symbolizing her maternal nature as the great mother and the "tears" symbolizing her role as Pity.
A female form
the first female now separate
Enitharmon as the first female form becomes the archetype of the female. This also marks the creation of the sexes, male and female, out of the androgynous Eternal.
his deathy face
the face of Los
The design for Plate 18 depicts the anguished Los looking at the female form before his face.
Petrify the eternal myriads
The sight of this division horrifies the Eternals to the point of inaction. The use of "petrify" also suggests that it begins to draw them into Urizen's stony sleep. Thus they flee from Pity which will also divide them.
Spread a Tent
In an act of horror and self-protection, the Eternals close off Urizen's and Los's world from their sight.
Science
Stevenson suggests that the tent is the restriction on perception that reason places on the investigator. This mode of rational and methodical investigation falsely limits thought and understanding (261n).